browser rendering

Possibly this issue is for other fora, which you might direct me, anyways;

I have been dablling making my own little webpages, however having gotten a 
nice
result jon fireforx, I realize picture sizes gets treated very differntly on 
different browsers !!!
so the looks of the pages get very strange from smaller (Opera) and much 
bigger (Explorer)
brower !!!

any hints for mitigation ?

br georg