Use a good quality graphics converter to save the image as a jpeg. Many programs(i.e. paintbrush) that save jpeg, only produce a working jpeg image, the compression is usually bad at best.
Gimp is a great program for cropping/converting images, it will let you adjust all of the various settings, and get the size/quality exactly right.
102400 bytes is 100k. Unless you're using something with good jpeg compression, most images of screenshot size will come out well above the limit.
