Remember that, when you put photos into Premiere Elements they become video -- and video resolution is only the equivalent of a 640x480 pixel image. That's why there's no point in loading extremely high resolution photos into a Premiere Elements slideshow. It does nothing but overload the program.
Per my books and the FAQs to the right of this forum, the ideal resolution for photos loaded into Premiere Elements is 1000x750 pixels.
If you want your photos to stay high resolution and you plan to play your slideshow on your computer, rather than on a DVD, you may want to look into a dedicated slideshow program, like Pro Show Gold.
I'm only intending to show it on computer/projector so I think I'll check out ProShow Gold or Producer.
Also, I'm planning on doing zooms and pans. Is 1000x750 pixels still the optimal Premiere import size if I'll be zooming?
This ARTICLE will give you some tips on handling large still images in an NLE (Non Linear Editor). You will get better quality if you do the Scaling in PS/PSE, before you Import into PrE.
Also, the viewed quality will depend on your Export settings, the format, the CODEC used, etc. Remember that if you are going to DVD (SD), then your images will be 720 x 480, so if viewed on an HD TV, they will only reflect that resolution. That is the nature of DVD.
To get closer to what you shot, you will need a BD Project (HD), and then burn to a BD.
Is 1000x750 pixels still the optimal Premiere import size if I'll be zooming?
For an SD (Standard Definition) Project - yes.
BTW - welcome to the forum.