Photoshop is a great and powerful image editing tool, but what if you don’t have a copy? There are two great alternatives: GIMP, and Paint.NET. While neither is a full replacement for Photoshop, the average user will never need the “missing” features. I use GIMP regularly, and no longer have Photoshop on my computer at all.
GIMP is a cross-platform (Windows/Mac/Linux), free image-manipulation program whose goal seems to be to emulate Photoshop as nearly as possible. If it isn’t a full replacement for Photoshop, the average user will never notice. If you have been using Photoshop, it may take you a little while to learn where things are (it took me a while to figure out where to adjust image levels, for instance), but they are all there. (If you don’t ever use image levels, you don’t need Photoshop–or GIMP, for that matter–anyway.) GIMP also includes a raft of filters and effects.
Paint.NET started as a free-time project at Microsoft. It requires .NET to run, and was originally meant to be a “free alternative” to the well-known MS Paint that has come with Windows since at least Windows 3.1. Unlike regular MS Paint, however, Paint.NET includes some filters and image effects (if you haven’t gotten tired of giving your little brother googly eyes). More importantly, Paint.NET uses layers and history (like GIMP).
Either GIMP or Paint.NET will allow you to do the image cropping, resizing, color adjusting, and goofy effects that you need to do. If you are a more advanced user, by all means, spring for Photoshop. Otherwise, these free alternatives should suit you just fine.