I use a free javascript called “pop-up images” that I got here: https://www.crays.com/jsc/
It works in IE and FF, and Opera for that matter. I’ve used it through several releases of WP without a hitch. You can see it on WP 1.5 on my site at https://www.persistentillusion.com/blogblog/index.php Just click on any of the pictures in posts to see the pop-up. I also have it running on WP 2.0 on my test bed, but that’s not publically available.
I installed it a little differently than the instructions suggest: Instead of pasting the script into my header, I created a file and stored it in a script folder. I then called the script in my WP header.
I skipped step 2.
In step 3, I do not use the onMouseOver and onMouseOut parameters in my posts when including an image.
For step 4, I put pimg.htm in my WP root.
I know that sounds cryptic, but it will make sense as you read the instructions.
note: edited to fix link. Oops.