Graphics in fluid boxes

I was wondering if anyone knows how to or if it's possible to place 2 graphics in one fluid box, one behind the other, to appear and reappear when needed without the fluid box shrinking both of them to fit.  I have 2 photos of a woman in my presentation.  They are duplicates except that the first one fades out and the second fades in with a slight variation of the first photo.  It works fine in older modules without fluid boxes.  When I try to put the photos in a fluid box it shrinks both of them smaller than I want.  Any help would be great.  Thanks.

David