Cabulb: While I personally am using GIMP, it should be similar in most programs
I just cropped the image using a rectangle selection a bit smaller than the faint visible outline. to further reduce the dimensions, I used the "Autocrop Image" option. This removes unneeded transparency from the edges.
Most likely, the outline occured in your case because the selection tool you used had the "feather" option activated. A selection feather is actually blurs the outlines of the selection. Kinda like the edges of a black rectangle on a white background, after Gaussian blur has been applied.