We're often asked for recommendations on image editing software that doesn't cost an arm and a leg. Photoshop is fantastic, but the price tag is steep, and in fact, its strengths can also be weaknesses in that it's actually such a powerful program that it can overwhelm the novice or intermediate user.
So, where do you turn if your needs are relatively straightforward and your budget is limited? I would recommend one of the following tools:
Picnik is a powerful online application that allows you to upload & edit images on the fly. There is a free version but the options are rather limited; however, it might do the trick for what your client needs. The premium version starts at $2.08/month. Picnik even integrates with Flickr, Picasa & Facebook, so if your client uses those tools that might come in handy. I find it a little slow, compared to Photoshop (mostly because of the time required to upload & download files), but it's a very good tool for people with no/little budget.
This is a stripped-down version of Photoshop that's perfect for what most of our clients need, which is basic photo & image editing tools for web use. It's under $100 and provides a good chunk of the core Photoshop functionality -- in some ways I would recommend it over Photoshop for basic-level users, because with fewer options it's less confusing.
Adobe's answer to Picnik is an online editing toolset that you can use for free; the only limitation is file storage. The free accounts comes with 2 GB of storage space for your files, and beyond that you pay an annual subscription fee that varies (anywhere from $20-$500 USD) based on the amount of storage you need.