Using FLASH
Good advice, jpinMilwaukee, to use mouse rollover, and any of the alternative images can be GIF89A animations as well.
I too avoid using Flash except when necessary, but consider this.
Macromedia claims 98% market penetration for their Flash-4 level plug-in. Whether this is believable or not is a question itself, but that's what they claim. At level 7 Flash it drops to around 50%. I sure don't want to torment my visitors with a need to download anything, so I try to use level 4 whenever possible. I've forgotten the penetration of level 5, but think it was stated to be around 90%. Level 5 includes nice alpha transparency.
Here's a problem I see, and it bears on use of Javascript rollover.
According to "thecounter.com" which gives us valuable insight into what our visitors will be using for screen res, browser, operating system, and more, the usage of Javascript is only 90% --
http://www.thecounter.com/stats/2004/September/javas.php
This is not a plug-in problem, but more likely foolishness of people fearing Javascript, plus some people who have primitive browsers, and others.
That means (if we believe Macromedia) that Flash-4 is more commonly available by 8% than Javascript!