There is no random pan and scan feature for Premiere Pro. I find that tons of stills and photos without proper memory slow Premiere and Encore. It's something that I wold like us to improve on in the future.
As a general rule here's how I view the apps for montage
Encore - Good and Easy
Premiere Pro - Better but harder
After Effects - Best but hardest.
Hope this helps.
I did a montage in AE and dynamic linked the comp into Encore. Encore would not transcode it until I removed the AE comp from the project. The only thing that worked was a montage from Premiere
For the Pan & Scan, I use the fixed Effects, Motion>Scale (Zoom) and Motion>Position (Pan), and keyframe as is dictated by the image. Some SlideShow programs, like ProShow Gold/Producer, offer Presets to randomly do these. Easy, but there is no AI to tell it what I think the image needs.
If you are doing a photomontage in Pro make sure the images are no lager than twice the sequence settings. So if you are doing SD that would be about 1440x960.
But if you have a Photomontage program like ProShow i would go for that.