The obvious inexpensive alternatives would be Pixelmator or Adobe Photoshop Elements.
The Gimp isn't really a native Mac app - and whilst it's powerful you may find it rather unfriendly in use. It does, however, have the advantage of being free. Personally I use the full version of Photoshop, but if I had to compromise, Pixelmator would be my choice.
I switched from a PC to a Mac earlier this year and Paint shop Pro 6 was my image editor of choice. You will be surprised to learn that there aren't any affordable Mac alternatives to PSP8... there are a few options that are reasonably priced but don't offer all of the features that PSP8 (or even PSP6) offered.
The two that I have are Pixelmator and Photoshop Elements. Pixelmator is the one that "feels" more like Paintshop Pro. When I absolutely need a function from Paintshop Pro, I fire up PSP6 within Parallels.
Oh, for vector illustrations, I use InkScape. It is free and does a very nice job.