Fuli, which version? i don't think it matters too much, just some thing may not be where i say...

Do your digital jiggery in Photoshop and jump to ImageReady.
You should see the same palettes that are available in PS.
When you have the Animation (Windows > Show Animation) palette open there is a built in TWEEN function that has someparameter that allow to adjust the opacity and the number of frames for this to take affect on (the number of frames will impact the level of opacity i.e. higher the number lower the opacity change). You can apply this to all layers OR to the "selected layer" (this opens up some creative options)

Another palette you will want to experiment with the options available is the "Layer Option/Style" (Windows > Show Layer Option/Style) This also has the "opacity" adjustment slider...

Try using some of these and see if you can get the effects you're looking for.

RetroCity