Photoshop is huge, especially if you're just resizing images for the web. If you're boss doesn't care about cost, than Photoshop is a good investment for other reasons (especially in the graphic to print area). However, Illustrator is also great in terms of web graphics--but very expensive too.
So a compromise might be Corel's Graphic Suite, where you get Image editing software (Photoshop) as well as vector/illustration software (Illustrator). GIMP is fine to, as are several other low cost options. you wouldn't believe the amount of people who buy a full version of Photoshop (for home or office) and don't use 80% of the features.