You've got two options, the file out of the camera can be reduced thus lowering the quality of the image, or you can alter it with some post processing software (GIMP is free), I would always suggest the latter even if it is a bit more of a faff as you can always keep the original (I'd recommend RAW which will be about a 12mb file) and then tinker later.

If the file is purely for the web have a look at the newer file format webp from Google gaining appreciation

A new image format for the Web | WebP | Google Developers