There are almost no tutorials for this game's mod support so sorry about that.

Assuming you have no idea where to start, create a file path like so:
C:\Users\YourName\Documents\Robot Entertainment\OMD2\MyMods\Testmod\art\

Then open 'data.zip' in C:\ProgramFiles (x86)\Steam\steamapps\common\orcs must die 2\

Look at the art assets under the 'art' folder. They have a .dds file extension.

Copy over the .dds files into Testmod\art with the exact file path of the image (so if you found the image in art\horde\orc\ then place it in Testmod\art\horde\orc\).

Open the copy of the DDS file in GIMP or photoshop and edit it as you please. When you enable the mod and load the game it should show your changes.

Another thing: If you want to use GIMP, you must install the .dds plugin from the internet. I had to look around for a bit until i found a compatible version.

Sorry to WAY necro this topic. But yeah I'm a new modder to this game and I also want to texture mod this game. I have followed your steps EXACTY how you spelled them out and I am having troubles getting the textures to show up in game. I can see my mod as an option when I fire up the game but my edits are not being shown... I thinking it has something to do with how I am saving the .DDS files in Photoshop and using the wrong compression. What kind of compression is are the files?

Yeah I'm a HUGE fan of this game and keep coming back to it. I'm a workshopper for TF2 and have learned how to do 3d art through that. Well now that I know my stuff I was thinking that modding this game would be fun. I know how to edit normals and sculpt and stuff so I could prolly do some interesting things with the textures. SO figuring out how to edit this game would be nice.

Well this was the last topic i thought would come back to haunt me - especially almost a year after i contributed to it!

Honestly, i never cared as to how the .DDS files were compressed, they never created any issue for me *in the way you described*. If you are seeing the original (default) textures rather than your customized textures in-game, then a few things could have happened:

1. Your files aren't in the right location
   
2. Your mod isn't active or is being overidden by another mod (check this!)
   
3. Wrong file extension/maybe compression issues?? (though i've never known that to be the case) I doubt this as OMD2 generally crashes without warning when it encounters even the smallest errors.

If you aren't seeing anything (no textures on models/replacement error textures) then it is surely a problem with the files themselves. The default export format worked for me + generated mipmaps aren't a necessity as far as i know