Hi cathyn518,
The max image size that we can display as a supersize image is 1280 x 1024 pixels. You can upload an image that is some bigger and it will be automatically downsized to 1280 x 1024. If you can, keep your picture file size to 1 MB or less. You can achieve that by using pics of no larger than the pixel dimensions I just gave along with your JPEG set to 72ppi (pixels per inch) resolution. Keep the 'quality' setting of the JPEG on the low side.
If you have a recent digital camera, most if not all the above can be achieved by selecting one of the small jpeg sizes available from your camera menu. Models vary, so you will have to read your manual, and maybe experiment a little.
The above numbers for the pics you upload are not absolute...it's more of an 'ballpark' to aim for. You can go some bigger on your picture files and the resizing on our site will take care of downsizing. But, best results can be obtained by staying close to the site display maximum as I just explained.
Hope this helps.