Pop up image with transparent background

[Martyd]

I've been using the css for pop up images (on mouseover text), but can't get it to work when the image has a transparent background. I'd like, e.g. to get a tarantula--in sillouette only--to appear on the page when mouseover text. Can anyone help?
Thanks

[bluewalrus]

Just show up in one spot and not move, when the mouse is on a specific text selection?

[Martyd]

Correct. I tried setting the background-color to transparent on the header html (everywhere I could), but I still get a big white box behind my supposedly transparent background gif.
Thanks

[Martyd]

Never mind. I figured it out. It only works if I load the image onto a separate host and link to it, then set the "span style" to "background-color:transparent" (which I can specify for each image individually). yay
Now I'll try an animated gif...