Artists Guide

There are two main areas where we artists can contribute. We need 3D models and textures for the same and the landscape.

Textures

We use the CC-by licence for OpenClonk and ask you to license your contributions under the same license. As many textures in texture archives have licenses which are not compatible with CC-by, we will have to create many textures on our own. But don't worry, it's quite easy and there are some really good tutorials on how to create quality textures from photos:

How to create seamless textures

There are different methods how to create seamless textures, here is how I do it: Tutorial: Creating Seamless Textures.

For GIMP, there are two filters which can do this automatically: The plugin Texturize and the filter Resynthesise. Both work good with textures with small random patterns like gravel, plaster, sand or ground. However, they don't produce quite as convincing results for bigger regular patterns like (roofing) tiles, bricks, wickerwork, pavements etc.

How to create normal maps for textures

From photograph to normal map, how to... - Excellent tutorial including tips how to photograph right
Creating Normalmaps - Very practical but little background information

Texture archives

Free / no Copyright / CC-BY

Here are some links to archives which offer textures "without any copyright", compatible CC licenses or public domain. Attention: If you want to extend this list, note that "free" or the lack of any copyright message doesn't mean anything. To be public domain or "without any copyright", it must be explicitly noted.

Flickr - mixed CC licenses
National Biological Information Infrastructure - good for mineral textures, public domain
LuGher Texture (3400+) - free to use in personal and commercial projects
Accustudio (~1000) - free of any copyrights
The Texture Barn (~500) - "Royalty-free textures for computer graphics, illustration, game design and everything else"
Texturenwelt (400+) - mixed licenses: see single images
Ben Cloward (~300) - free but author would like to be notified where it is used
Free3dsTextures (~200) - mixed Creative Commons Licenses - Attribution license would be compatible
Blender Texture Library (~200) - Creative Commons Attribution License 2.0

Mostly free with own license

The most and the biggest free texture archives offer the textures free for any purpose, but it is not allowed to redistribute them as a texture pack. So, in all cases, we need to print the license and copyright message with every redistribution. As this licenses for textures from these archives are incompatible with the CC-by license, we don't favour textures from these sources. However, we can still include them as long as we mention that these textures may not be freely redistributed by default. Before you contribute any textures of these archives, carefully read their terms and conditions as they all differ from one another:

CG Textures (30000+)
Image After (~10000)
Mayang's Free Texture Library (3800+) - max. 30 textures may be used
2Textured (2700+) - "The textures may be used freely in commercial or personal works but reselling or redistributing of the textures on other texture sites is forbidden"
textures.pedramk.com (~1000)
Freetextures.org (~1000)
Texturez (800+)
Free Texture Site (600+)
Back of Beyond (~500)
Zen Textures (450+)
Noctua Graphics (400+)
Texture King (300+)

Modelling

The size in which the graphics are displayed will be about 3x as big as that of Clonk Rage. Additionally, it is planned to directly render the bigger models in game (real time rendering), most probably with OGRE.

Models for real time rendering

Now, if you are used to create models to render them afterwards, you need to take into account a few additional things:

The models need to have a low polygon count. Details like e.g. rivets which you perhaps used to model before are drawn in the texture. The texture is a very important part of the model. Thus, creating a very good and clear UV map for your model is a necessity (Tutorial for blender). It not only affects the quality of the texture map, but also of any other maps like normal maps or reflection maps.
The guideline on how low-poly the model should be is: The model with texture still needs to look good when displayed 3x the size as in Clonk Rage - there is no fixed number on how many polygons are good since it depends strongly on the model.
You should delete the faces that are not visible from where the model is looked at in the game. These include faces that are just not visible from any direction because they are inside other objects but also faces which will never be visible in the game. For example the backside of a house or the bottom of most models will never be visible. However, this process is very fast when you do it when you are done with the model - just before you create the UV map. So don't worry about it while modelling and also don't overdo it - A few polygons on the back of the house do not matter so much.
Always start with the low-poly model, it's much easier to start of with a low-poly model then to first create a high-poly model and then get it down to a low-poly count. You can always add details later, either in the texture or in a high-poly model. In Blender, there is a option called "multires" which allows you to create several models in different detail levels based on one low-poly model and it's geometry. Baking normal maps (Tutorial for blender) with a high- and a low-poly model only works if both have the same geometry.
Creating normal maps for models only makes sense for those which have organic, round parts where too much polygons would need to be used. So, for buildings it's normally not necessary - more for living creatures with one (Blender: example) enclosing mesh.