<p>I'm looking for some image library that can work with multiple formats (jpg,gif,png), and is fast at displaying the images to the screen.</p>

<p>Also, it would be nice if I could specify only part of the image to render to the screen.</p>

<p>In the end I want to have lots of images on the screen that I can pan and zoom about.</p>

<p>This is for a personal project on my ppc powerbook, and I'd prefer if the library be in c++.</p>

FreeImage.
That's for loading.
For displaying, you can either use the FreeImage methods to make a bitmap that you can then display in some control (depending on your UI).
For the real hotness in display, you'll then want to use
OpenGL
.
Also, in the meantime, I've discovered
CImg
, which isn't a library so much as a framework for doing lots of common imaging applications and works in C++.