When providing guidance to clients who are selecting images to use on their sites, there are several factors to consider. WordPress will create thumbnails at the appropriate cropped sizes (as defined by the theme developers) where they're called for in the design but in general:
File Size: 1MB or below (you can upload images up to 2MB in size but policy-wise 1MB is a better goal)
Resolution: No more than 72dpi (that's the maximum resolution images can be shown at on the web in general)
Dimensions: The largest dimension (either width or height) WordPress will accept is 2560px, so anything over that will get automatically cropped on upload down to that size.
Once the final designs are complete a list of every thumbnail size the site would use will be available, but in general you want to load the largest images as possible in terms of dimensions to match the biggest thumbnail size. As long as the first three points above are considered, though, they'll be fine for selecting and uploading images.
It's generally better to have WP scale the images down (taking a 1024x968 image and putting into a 500x400 container), rather than asking it to scale them up (taking a 500x400 image and trying to fill a 1025x980 container).
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