Are you on a PC or Mac?
I haven't used the application but "GIMP" is freely available for both platforms,
GIMP WebsiteOne thing that needs to be considered is "image resolution", this will almost always need to be reduced as anything above 72 DPI is wasted since monitors can only display at 72 DPI resolution. Since you may be attempting to use a photo from your digital camera and this is set to a high resolution setting, you'll quickly exceed the 100,000 bytes(~100KB) limitation.
With this said, the first thing that will almost always bring a file size way down is changing the image resolution down to 72 dpi. From there, the next issue would be to shrink the image dimensions. Depending on the application, you'll want to ensure the dimensions can be "constrained". This will allow you to change one of the dimensions and the program will automatically change to the other dimension to keep the image from distorting. My recommendation would be to resize down to something around 640 X 480 in pixels not inches since the web is defined in pixels where as the print world utilizes inches(for the most part...). So, if you're seeing inches as the unit of measure, you'll need to change this setting; possibly in the options, settings or preferences dialog box.
Hopefully helpful...