I've played a few MP games in Crysis Wars where there were custom pictures in the game, is there a tutorial anywhere on how to do this?

Pretty easy. All you have to do is this:
1) On GIMP or Photoshop, scale your picture to the correct dimension. (256x256) (512,512) (1024,1024)
2) Save the file as a TIF file.
3) Drag this tif file into your materials folder in your crysis install
4) Make a new material and set its source as your file.

How do I add a source? I got the TIF in materials and I got the new mtl file ready. But I can't seem to get the Source working.

EDIT: It says something about an xml file error

I'm going to try and download Crysis War MP missions with custom textures and see what they're doing with XML files.

I suggest you take a look at the documentation:

http://freesdk.crydev.net/display/SDKDO ... ial+Editor
http://freesdk.crydev.net/display/SDKDOC2/Texture+Maps

You don't need to do anything at all with XML files, nor download custom mods just to see custom textures (they work in exactly the same way as regular textures).

All you need to do is make your texture file in DDS or CryTIFF format (regular TIF might work too if you don't have the Cryengine plugins), save them in a folder of your choice (for example Crysis Wars\Game\Levels\MyLevel\customtexture.dds) and browse the "Diffuse" slot in the material editor for the file you just saved.

You can either select an existing object and edit its material, or create a custom material by right-clicking somewhere in the "Materials" folder in the material editor window and selecting "new material".

Ah, well theres the confusion. Are you trying to make a custom material or a custom texture?

A material is the visual skin you apply to an object. It is flat, and used for skinning and simple decals (like graffiti)

A Texture is the, well, texture, of your object. Examples are things like grass and asphalt. These are used in conjunction with materials to create highly detailed objects (mainly terrain painting) You don't have to mess with textures to make a material image ingame.

Since you are talking about importing images, all you need is material. Do do this, follow the steps I outlined above. To make this easier, I will share the easier method.
1. After verifying that the TIFF file and the MTL file reside in the materials folder.
2. Go into the material editor and find an existing material that is similar to your intended image.
(if you plan on making say, a word painted on a wall, use a graffiti material)
3. Duplicate this material, and rename it to your custom name. Save it as this name when prompted. (Make sure the save location is in the materials folder! Sometimes mine defaults to My documents)
4. Afterward, simply change the diffuse material to you own custom material, and you should be done. Depending on how big your chosen file was (256x256, 512x512,1024x1024) You may need to do a bit of scaling.) Occasionally you may need to tinker with the colors bar too.

This should enable you to set images a MTL files in game.

I'm trying to get a picture in (like a decal/graffiti)

So far the TIFF file isn't working (I made it 1024x1024 via GIMP, saved as TIFF).

Made the material for it. And put it in the diffuse, but it's still showing up as a blank grey thing.

What loses me is when you tell me to

"3. Duplicate this material, and rename it to your custom name. Save it as this name when prompted. (Make sure the save location is in the materials folder! Sometimes mine defaults to My documents)"

I followed the one tutorial the other guy posted and the image is still showing up as a default grey texture.

Are you sure you apply the material to your Decal object?

+1

Okay, say you find Material X, which is say, Oil Spill.
What you do is go to that material and right-click and hit Duplicate Material.
What you will get is Another Material X in that folder. Right Click the second Material X (it won't have a padlock next to it) and Rename it to Your Material. It will ask you to save it to your hard drive. Like I warned, sometimes it will default to your documents folder. Make sure it goes to your materials folder.
After that, apply the diffuse of your material.
The reason I use this method, is it makes a template of sorts, and its harder for something to go wrong.
I've never tried to use a material 1024x1024. Perhaps it is too big. I've always used 512x512 and never had a problem.