How well an photo will compress depends on the content of the image,
lots of sky or an photo with the background out of focus will compress
more then a photo that is sharp and has lots of detail, like grass,
sand, trees.

Normally 100K would be good for about an 640 x 480 image but might be
able to go up to something like 800 x 600, this depends on how much
detail is in the photo. The best thing to do is to try saving it at
different sizes and see how they look. Do "Save For Web" this
strips off the EXIF data which will save some room and it allows find
control on the quality used to save the photo.

Scott