Open Pan/Crop, right-click in the centre and select "Match Output Aspect".
This will get rid of the black bars.
The other thing you should do is resize your images to no more than 2x your project size.
If you're doing deep zooms, then leave them at their original size, otherwise reduce the size as this will make your project run smoother.
IrfanView is a great and free tool for doing this.
It has a batch resize tool that will process over 100 images in less than 30 seconds.
http://www.irfanview.com/