We have been trying out an open source tool for Windows called Paint.Net. It lacks much of the sophistication of photoshop, but it's all you need if you need to crop, resize and perhaps alter the tint of photos for use on the web.
It's easy too - if you've tried any other graphics editors, you'll find things in familiar places (select an area and then click "image", "crop to selection" to crop, click "image", "resize" to resize.... not much can go wrong).
It's a little awkward to download. You have to go to "getpaint.net", which isn't the first result in google, then click the download link and then a download button - and then unzip the download before running.
It's very worth the trouble though - try it and let us know what you think!