You edit the texture files in an image editing software capable of working with .dds files. Photoshop and such.

As for the minidump, without knowing the exact circumstances of the minidump its hard to tell what it might be. Generally speaking, the first step in bug-hunting is to try and reproduce the crash. That will tell you where to look... I can tell you right now it's not a major error since you can actually load the mod.

Middle-of-game dumps are usually caused by incorrect entries for various .entity (misspelling an .entity file name in, say, build lists for a given race will result in a dump only when you mouse over the corresponding icon) files or errors in secondary files like particle effects; for example, if you make a particle effect which has an error in it, it is likely you will get a dump only when the game tries to actually render the said particle.