All images must be 8 MB or smaller. If they are too large they will not upload at all, if they are too small the image will be blurry. 8 MB images have a resolution of 1600 x 1200. If the photos are larger than this they will not upload.
Please also avoid uploading photos that are smaller than 600 x 400 as these will not look good on the website. Do you want to see the difference?
Here is a photo from Wikipedia that has a low resolution: 300 x 201. The photo is a little blurry. If the photo was enlarged any further it would be very blurry.
This photo was taken at the 8 MB range 1600 x 1200. You can see how clear and crisp the image is.