Desperado

Either replace the textures.big with a back-up, or if you made a back-up of the individual texture in question replace that. This link tells how to replace a single texture:
http://forums.projectego.net/re-texturing-dummies-3558/
If you didn't make a back-up, check my sig for a link to how to get the original of the disks, second spoiler tag.

Make a copy of the fmp, then rename it. Change the end .fmp to .big , then in FE click open, then the .big. It will appear in the box in the upper left of FE, double click it to view the entries.