This can have a big effect on the speed of a web page and surprisingly many otherwise good quality web sites contain images that are larger and hence slower than they need to be.
Step 1: reduce the image dimensions in pixels
In OS X the easiest method of reducing the image size is to use Preview (or Paint or similar on Windows) to resize the image to the same width as the maximum width of the page, say 1000px. If a portrait orientation image is to be used in only 2 column mode then it can be reduced to 500px on the width.
Step 2: optimise the image file
The best tool on OS X with which to reduce the size of images by optimisation that we have found is ImageOptim. It is fast, efficient and free with a nice request for a donation which it seems to very much deserve.
Typically it achieves reductions of between 10-30% in pngs and jpegs.