here's a better way to do what pg said:

<img src="whatever" :bou::bou::bou::bou::bou::bou::bou:over="this.src='whatever2';" :bou::bou::bou::bou::bou::bou::bou:out="this.src='whatever';">

i.e.

<img src="http://foolsgold.kicks-&#97;ss.net/forum/images/smilies/ezpimp.gif" :bou::bou::bou::bou::bou::bou::bou:over="this.src='http://foolsgold.kicks-&#97;ss.net/forum/images/smilies/hitlartime.gif';" :bou::bou::bou::bou::bou::bou::bou:out="this.src='http://foolsgold.kicks-&#97;ss.net/forum/images/smilies/ezpimp.gif';">

ok.

but yeah, more or less you'd have to learn flash. or divide it up into lots of little tiny images, which would suck.