Take an image in photoshop and place a second image on top of it as a new layer.
At the bottom of the "tools" pallete is a little icon that will cause you to "jump to " "Imageready" a web prep program built into photoshop that about half the people I know don't even know is there.
Under one of the main menus (I think it is the "Windows" menu up top) is an option to show the "animation" tool bar.
Click that. Your main image will be displayed on screen, just like in photoshop, and the "animation" pallete will show a small thumbnail of your image (it looks sort of like a layer's pallete but flows from left to right).
Click on the "new" button on the animation pallete and you should now have 2 thumbnails of your original image.
Click on the second image (the image on the animation pallete coincides with what is on screen). Go over to your layers pallete and reduce the opacity of the top layer to 0 which should make your bottom photo show now.
By clicking between the first image on the animation pallete and the second image, you will see your main image jump between the top photo showing and the bottom one showing.
Click on the first image on the animation pallete and then click on the little icon that looks like about 4 sticky notes overlapping side by side.
That will bring up the "TWEEN" menu.
Put the number 10 or so in the box and click o.k.
What "imageready" does, is fabricate about 10 images that are slowly transitioning between the top layer being fully opaque and the bottom layer being fully visible.
You will now see about 12 images on the animation menu.
As you click on each one, you will see the effect progressing.
You can hit the "play" button and watch your animation work.
Beneath each photo on the grid, is a time designator.
You can click on that and change how low that image shows, like .5 seconds for the first and the last.
You can also then go to the last image and "tween" back to the first image so it continuously rolls over.
Go to the main menu and click "preview in browser" to see it work.
"save as optimized" will save it.
Up in the upper left corner of imageready is an optimiazation pallete that has various settings like 128 dithtered, etc.
This is sort of like your "jpeg quality" save as box.
The setting you put there will effect your output image.
BTW, you can do all kinds of crazy things with multi layer photos, or manually move layers around and things.
This is quite the powerfull tool, and this image is just the tip of the iceberg of what it will do.
For example, you could go to the each image on the animation pallete, and on the first one, type in the first letter of your name, on the second type in the second letter, on the third, third letter, etc etc.
As the image then scrolls through, your name would be spelled out incrementally.
Think of this like the way the do cartoon "cells" and you will get the idea.
You can have a background layer, and each layer above that is like acetate that you can draw on and move around, so you could have a steady background but move the roadrunner from left to right, if you wanted.