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If its a photo/jpeg, you can experiment with applying greater levels of compression. You'll need an image editing program. If you are lucky enough to have a recent version of Photoshop, it has a great 'save for web' features that lets you compare file size/appearance of a photograph under different levels of compression.
If its a GIF or PNG you're dealing with, you can experiment with reducing the number of colours/bit depth in an image (again, you'll need an image editor). Usually you can dump quite a few before the image starts to deteriorate.
You can get huge savings in file size if you're prepared to play around a bit.
