I fixed this, and I am ashamed that it is the easiest fix ever : just copy the *image.xp3 files from setup to the install, overwriting all of the existing files. It now works flawlessly.
The solution was actually so simple I'm ashamed, haha. But if someone else is ever coming here with the same problem, this is the solution:
I had the .rar file of UBW still saved so I unzipped it, copied the two image files (fgimage.xp3 and image.xp3) and simply pasted and rewrote the original game folder's same files with those.
Thank you for your help!
I probably would have reinstalled the whole game if I didn't know the problem was just in the image files.