I usually just experiment with the push/slide/pip transitions. It's a pain figuring out their application and necessary tweaking to get the result you want but, after that it's life on easy street.
Also unless you're gonna go get some 3rd party plugins you might just be in for a keyframe of a time.
Thanks. I was hoping someone may know of a ready made plugin that could achieve the effect. So it looks like keyframes here we come, although I don't know about getting "floating" effect.
Once you have set your keyframes save them as a preset
That is a LOT easier in After Effects.
However, if you put the stills in a different sequence so you can put a little white background around them, you can then use the nested sequence as a slide that can be set to motion with keyframes. And as stated, you can copy and paste the keyframes to get you to the same place each time, then just slightliy adjust the ending location to create the illusion of a pile.
Or, to skip the nested issue, use Photoshop to put the border around the image, perfectly sized for your needs before you bring them into Premiere Pro.
However, if you do use the nested sequences, some of the images can be stills, but then some of them can be video, that can then be set to run normally, within the framed border. Just a thought. Start with one copy that has a hold frame at the begining and then replace it with the running video.