The themes for firefox are JAR files, compressed archives made with apps like Winrar. Use winrar to open the ".jar" files and look inside the folders of the compressed file for tuning the graphics. U can make an old skin work in early versions too.
There is a lot of info regarding that in some forums at mozilla site.
I can assume that Mozilla Firefox reads these skin images from these files. Try browsing around in there and look for the image you want to edit. Replace it and update the archive.
Btw, what kind of image were you wanting to replace?