Modify File Size

Probably, but take a look at the specs and at the compression to see where it's safe to add data (if it's safe at all).

I'm on another PC atm.
Why can you modify the file size perhaps?
I'm just testing it out on an edited .dds file of the skybox folder.
Once I now how to edit it, I'll start with the real deal

Yea I came across this problem as well. I was trying to change the color of the uniforms and still am. I found that if you can brute the check sum, and make it correct, but this adds 4 bytes, (if i remember correctly), which sets off the error. So I took the edited file into a hex editor, and removed the last 4 bytes, (which broke the file), then used the brute check sum, and replaced the check sum with the proper one, which in turn replaced those missing bytes. (which oddly "fixed" the file)... I thought I had it in the bag, Same image resolution, same bytes, same name, same everything.... Nope, still got that pesky check sum error. If anyone can find the answer to this, you'd be my Hero.

I didn't receive any error but the image just doesn't display ingame :P