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 size
Step 2: optimise the image file
In OS X the easiest method of reducing the image size is to use Preview 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 shorter side.
The best tool on OS X with which to reduce the size of images by optimisation that we have found is Optim. 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.