If that doesn't help, delete/move the flash03.tga file from the
Maps folder altogether - this will force the game to load the default texture. I think I had a similar problem years ago related to TT's texture pack I was able to solve similarly.
IIRC this game makes things look transparent by putting a mask over the textures, since bmp files on their own don't support transparency. If you look in your Maps folder or your Maps_U folder, there should be another texture with the flash file with the same name apart for an "
op" or an "
a" appended to the end. This will appear as a black and white silhouette. See the wheel of the Bedford as an example:
Its possible that if its not in the folder the game isn't loading the vanilla mask. If you cannot find this file, you can try unpacking it from the game with a .dta extractor and copying it into the maps folder. With that in mind, unless you are specifically using a hi-res texture mod for the muzzle flash you will be just as well off using option a and deleting any muzzle flash files from the maps folder.