As you guessed it already, animationIMG.pspr is an animated visualiser image. It is an ordinary animated .gif file that has also been renamed into .pspr for convinience. The size of this image depends on what you're trying to do.
You have probably noticed the visualiser image is also included in the backgroundIMG.pspr but with no bars in it. This is because this area gets covered with animationIMG.pspr as soon as the stream starts playing and gets uncovered when the music stops.
You can place this animated image anywhere over the backgroundIMG.pspr, so make sure it has a transparent background when needed.
This is the example of a sample animationIMG.pspr image that goes with beforementioned backgroundIMG.pspr:
Word of caution. Try to make both backgroundIMG.pspr and animationIMG.pspr images as small as possible in terms of file size. PSP browser is notoriously low on memory, so if you do not want get 'Out of Memory' errors, optimise both images and don't make animations that are too large or too lengthy.