Always shoot at max resolution. Use your Canon. Figure out how to make it work for you. There's no sense in taking pictures at a decremented resolution to fulfill some arbitrary constraints from a website.
Irfanview is a wonderful, free program and I'd highly recommend it. It's pretty powerful for what it is and has a small footprint.
In addition, not only can you vary the size of the x,y dimensions as Gene mentioned, but you can also adjust the compression (the z-axis or color depth that I mentioned - or "altering the pixels" as Gene explained). See example:
By setting the quality to 27%, I managed to save an image that is 1000x677 pixels without a huge loss in quality - right at 80kb:
But the really huge question that you've not answered yet is at what resolution (pixels x pixels) are they going to be displayed. It would make NO sense to kill the quality (depth using this technique to decrement the quality) of a 4000 x 3000 if it were only going to show up at 200 x 150 on the website/screen. That's the first question in any of this.
