Just make sure that the new graphics file you are recompressing to the ROM is the same size or smaller than the original graphics file you decompressed. Once you decompress one of the graphics packets, copy it and make edits to the copy, then right click on the files and compare the sizes of the original file and the edited file.
If the edited packet is larger by even 1 byte, things go badly (you are overwriting something right after it).
Also, make sure you are recompressing it back to the exact same address you decompressed it from.
Or... Repoint that packet to some freespace if it has to be larger, though TF uses freespace... I forget if the graphics packets pointers are 2 or 3 byte pointers, if they are 3, then it's possible to make it larger, and put it in a bank way past what TF will ever reach. I think that made sense. Sort of.
>_>