Not sure what steps you have taken already, but if you are using Photoshop or Elements, check to make sure you are saving
1) "for the web"
2) as a jpeg
3) and in addition to the physical dimensions, make sure you are saving at 72 pixels per inch.
There are different ways of doing all that if you are using a non Adobe product, like Picassa... If the above wasn't helpful let us know what program you are using so more specific advice can be given...
