To find our in-game images, (both for comparison, and to see your creations ingame), read these instructions:
http://www.wesnoth.org/wiki/EditingWesnothThings we need help with, if you're an Illustrator/Character Artist:If you're a traditional artist, able to depict landscapes, scenes depicting events, or portraits of an individual characters, we could use your help doing exactly that.
Portraits:Campaigns have individual characters, and we'd like one portrait for each (major) speaking character in a campaign. For generic units, which is how we handle minor characters, we would like to have a single picture for each unit line in the game (it's okay to share portraits between members of a line; for example it's fine for a "bowman" to have the same portrait as the "longbowman" he levels into (although we'll happily welcome unique ones for both).
We're shooting for a standard style; you can see this by looking at our current human and elf portraits. We're not interested in something obviously mismatching that style; it's fine if you're, say, a manga artist, and some of our user-made campaigns might be interested in the work, but we can't accept it into the core game. We appreciate your intent of helping out, but in order to look good, we have to reject what doesn't match. Sorry.

These are the detailed requirements:
viewtopic.php?f=9&t=24555Currently, we're doing fairly good with our generic units, and are down to just a few remaining units, but we could use a bunch of help providing, or replacing "obviously out of style" portraits for the mainline campaigns. This is a list of what to work on:
viewtopic.php?p=292889#p292889Story Images:Campaigns benefit from depictions of major events in the story. Currently, the only campaigns we have decent pictures for are Heir to the Throne and Two Brothers. We're considerably more relaxed about style when it comes to campaigns, but we still want only professional-quality work. Both paintings or line-art based work are fine; as long as they're well-done.
Things we need help with, if you're a CG Artist/Digital Painter:Terrain:A few of our terrains are out of date, and could use better treatment.
See
this post on Needed Terrains- Probably the worst offender is desert hills and mountains, which need to be completely redrawn. It's suggested to make desert mountains look like basaltic/volcanic rock (like in the sahara), or to make them look like the mesas in the american southwest.
- Our current "wildflowers on grassland" is very simplistic and could use a redo.
- Race-unique variants for castles and villages are welcome for every race.
- New terrains are welcome, but they need to be very well-drawn; we can generally find a use case for almost anything, or alias it with existing terrain, but it needs to look exceptionally good. We are interested in interior terrains, too; such as walls and furniture.
Attack/Item Icons:We're pretty much done, but campaigns occasionally call for new images, as do unusual multiplayer scenarios. If you make something good, we'll put it in mainline so everything can use it.
This style of icon works very well for 'commands' in a multiplayer scenario, such as one where you can buy better armor or such, or build stuff; the community is encouraged to use that style of icon, there.
Things we need help with, if you're a Pixel/Sprite Artist:As far as the core "mainline" game is concerned, we're not open to new unit designs; although it's a lot of fun to make new units, we've got a full set in the game right now.
What we need help with more than anything else is improving our animations. It's the best thing to do because it improves every player's experience, and because (as long as they're well-drawn) nearly guarantees the work will be included in-game. We really appreciate the help, and the lead artists are much more willing to use their extremely limited free time to teach people if those people are working on the core of the game, rather than on some peripheral feature.
If you're dead-set on designing new units, we have a lot of user-made campaigns which would love to use your work, but it probably won't reach as wide of an audience as making stuff for the core game.
Short Notes on Animation:With all animations, the best thing is to do it right the first time. "Placeholder" animations are not welcome, because they don't look better and someone has to redo them anyways, later.
Practically every one of these animations will need at least four or five frames, minimum. It's at least an afternoon's work or so. That said, a huge time-saver is to "clone" an existing animation - if an animation of the right type exists for a very similar unit, you can copy in appropriate body parts to change the animation into one suitable for the new unit. This is similar to the process described
here.
Attack animationsAttack animations should generally point to the SE direction. For bladed weapons, they should swing in a nice arc so that the same animation can nicely be used towards the S as well. Open and closed threads on the subject:
viewtopic.php?f=9&t=9163viewtopic.php?f=18&t=21512Defence animationsThese generally point in the same direction as the base frame, and will be 2 frames long - one frame 'preparing to get hit', one frame 'actually getting hit'. They play very quickly (over within 250ms) and therefore should never be longer than two frames. Many of the current ones are only one frame, so there is room for improvement there.
Idle AnimationsIdle animations are of the unit doing some action once they're sufficiently bored, such as kicking dust around, or swinging their weapon through the air. They trigger at some random time interval after the unit is inactive on screen:
viewtopic.php?f=9&t=12787Standing AnimationsAka breathing animations, like one would see in a fighter game. Besides winged-units flapping, we have none yet, but we'd like to have some.
viewtopic.php?f=9&t=12787Death AnimationsThese need to be about half a second long, which means about 4-5 frames. We're hoping to standardize these to all fit the same system, so that we can reuse some of the frames; this will mean that most of the animations should simply be of the character falling forward onto their face. The first frame should double as a "stooped stance from critical injury"; as if the character is on their last legs. The last frame should be of them lying forward facedown, and will double as a corpse frame.