If you're serious about digital photographs, iPhoto's tools by themselves aren't going to cut it. You'll have to spring for a more full-featured image-editing program, such as Adobe Photoshop, Photoshop Elements, Canvas, GraphicConverter, or a similar program.
Photoshop is by far the most popular tool for the job, but at about $600, it's also one of the most expensive. Unless you intend to perform sophisticated image-editing and compositing (sandwiching together parts of different photos into one image), save yourself some money and buy Photoshop Elements. It's a trimmed-down version of Photoshop with all the basic image-editing stuff and just enough of the high-end features. It costs less than $100, and a free trial version is available online. (Your digital camera may even have come with Photoshop Elements right in the box.)