Do you have photoshop?
Look under "file" for "save for web", pick JPEG Medium from the pulldown list and resize the picture as necessary. If you look towards the lower left corner of the save for web window, it will tell you the estimated size before you hit save, so you can reduce a bit more or crop if necessary.
If you use another program to do the reduction, the concept is to not save any of the EXIF data with the new file, no header data, and reduce the JPEG quality to about 30 (you might have to look in the save options of whatever program you are using) this and a reduction in size to around 1024 or 1280 or less should get you under the 100KB limit.