First of all, you must unlock the base layer in the JPG file. After you do that' take the selection box tool and make a box that fits the edges perfectly well. Then take the background layer (Should be called Layer 0 now) and add a layer mask to it. That will take the edges off.
However, the corners are still there. To get those off it's best to manually erase the corners with the brush tool (Still having the layer mask selected.)
Then save the final file as a PNG or GIF, this is CRUCIAL to keep the edges.
Then upload it and you'll be good to go!