Photography and layout pros use high-end image editing tools when they work. Adobe Photoshop rules this particular category of software, to the extent that “to photoshop” has become a commonly-used term meaning “to alter an image.”
Photoshop is expensive, and for good reason – it offers many capabilities and saves plenty of labour. That said, you can perform many common, simple types of edits using much less expensive (even free) pieces of software.
You can, for instance, paste many types of images into Microsoft Word, right-click them, and alter them to suit your purposes in the Word document – no need for other programs here.
Mac users get a number of handy programs included with their computers, right off the shelf. One of my favourites is Preview. It’s a Swiss Army knife that I use for all sorts of things: reading and annotating PDFs, combining PDFs and so forth. The Mac even lets you generate PDFs from the standard Print dialog!
The things you can do to PDFs you can also do to images, all within Preview. Here’s a helpful video that demonstrates Preview’s image-editing capabilities.
http://www.youtube.com/watch?v=sJN8WXehIQs