Great question! The short answer is 600x600 pixels.
This size allows the image to be quickly uploaded, it allows the image to fit nicely in the image annotator, and it prevents the image from overlapping on your tech pack exports.
Troubleshooting:
If you're still having problems with the formatting within your tech packs try cropping your image to reduce the amount of white space on all sides of your image.
If this still doesn't work consider changing the format of your tech pack from condensed to expanded view to allow slightly more breathing room for your image.