Color
Changing or modifying the colors in an image can produce some very dramatic changes. There are two ways to do this: Color Editing and Color Cycling. You can also change the number of colors in your image by using different Graphics Modes.

Size (or resolution)
The size of a digital (computer) image depends on how many pixels it has. By choosing different Graphics Modes, you can vary the resolution of your image.

Miscellaneous
Pressing "z" allows you to change various parameters specific to the formula in use. This usually includes the real and imaginary perturbations of Z, the bailout values, and possibly a mathematical function or two. I don't know what most of that means, but it's all in the Fractint manual. When changing the functions, a single key press (the first letter of the name of the function) will bring up the function (for instance, pressing "c" brings up "cos", and "l" brings up log). Some keys have more than one function assigned to them - pressing "c" repeatedly will bring up "cos", "cosh", "cotan" and so on.

Pressing "x" (extended options) or "y" (more extended options that wouldn't fit into "x") allows you to change other fun stuff. Try everything at least once. Especially neat is "inversion" which can be found in the "y" menu. Inversion shows the image in such a way that if you looked at it through a mirrored cylinder, the image in the mirror would look normal.