There's a few solutions depending on how your comp is set up and with what programs.
For a graphic/artisitic person with photoshop, I'd tell them to change the resolution to 72 dpi and save for web quality (low res jpeg ect).
For the average Joe who thinks what i just previously typed was me speaking in tongues, you'll want to take your image and open it with MSPaint. Resize it down between 50-70% of original size, and save as a jpeg (not bmp!!). Quality still readable.
(<-- a graphic designer who was taught in school to keep images for web-use under 92kb)