The best resolution for a photo to be uploaded here is typically 640x480. Try to resize your photo before uploading, without overwriting the original.
All digital cameras are usually set by the user to take high resolution photos. While this is great if the user has intentions to use the photo for a poster, the high resolution photo is way too big for display on a web page. The photo's high quality is wasted. If full resolution photos are uploaded here, over time the storage space required will become an issue. For the purpose of displaying a picture on a web page, a 100kb picture looks as good as a 6meg picture.
To resize pictures you can use a program in Windows called Paint. There is a great Windows program called Faststone Photo Resizer that can rename and resize photos in the same operation.