1. The cropping techniques that I use in my pictures involve the magic wand tool, the lasso selection tool, etc. I also crop my canvas whenever I make a Flash animation.
2. Pictures optimized as jpg are more complex, more colors, any canvas size; more likely to be a photograph.
3. Pictures optimized as gif has less colors, much less canvas size; often an icon, or is animated.
4. Pictures optimized as png are less complex, more colors with less shades, any canvas size; may be a cartoon, clip art, logo, map, etc.
a. It's better to use Jpeg when the image doesn't need to be reused at all, or if the image is primarily a photograph. It has "color loss" which makes it so the picture is harder to edit, but easily compressed.
b. Gif's and PNG's have less color loss or even no color loss compared to JPEG, so those formats are great when you want you material to be edited, used, etc.
On iPlayWinner.com, you used to get this header, I liked it too, for its crown logo, representing the idea of winning in an arcade fighting game:
Here is a very illustrative one for VGJunk. I like this one, since it easily explains that it is a website about videogame nostalgia by being very pixelated over almost a Mario-esque logo:
