I can't remember exactly how to do an edge detection mask for a bitmap but it's something like

0 -1 0 -1 0 -1 0 -1 0

as a matrix applied to the bitmap, this has issues with jpeg compression though as you get square artefacts from the lossy-ness of the compression, soi it may be an idea to detect the compression window for the jpeg image and use that to scale the matrix.

I believe jpeg compression is more or less just an inverse fast Fourier transform.