How do i edit the file size of a picture, but keep the same perimeter..(for bebo skin)?

I use 2pic to make the main background image the right size for a bebo skin.. and i made a good one, but everytime i try to upload it, it says that it is 2 big...to be exact : main_bg file size is too large. Actual is 65912, maximum allowed is 40000, please rectify and upload again....i have the correct perimeters so how do i edit the file size.

How do i edit the file size of a picture, but keep the same perimeter..(for bebo skin)?
You need to change file format of the picture, or, reduce the size if you keep the same format.

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