There has to be something really simple wrong, because I do this all the time and it works OK. Admittedly I make sure all the clips are the right size to begin with.
Remove the background and see if the Tif stills resize again, I'm assuming they wont. This will at least tell if you if the problem is with the Tif files or the background. If it doesn't resize then put the 480 x 252 background in a new sequence and resize it, then place that sequence in the place you would normally have put the 480 x 252 background in.
Hope this is of some help.