You can't remove the white from around the explosions. That is where the image is transparent. Being an animated gif and it's in gif format which does not support transparency the only way to get rid of those would be to take apart the gif and make the edges of the image black so it blends in with your background. You'd have to do it frame by frame which would take quite a bit of work..
Or just make it all again from scratch and have a black background to match your forums background. Then it would look perfect.
If you'd like me to fix it, make a request in the
Graphics Request section and I'll get it done for you..