.PNG : These are perfect for non-animated pictures that need transparent backgrounds and have less than 255 colors. They are also good for pictures with more than 255 colors that don't have need transparent backgrounds. They're even better than static .GIFs because of their small file size and easy use with earlier versions of PHP. They can also have extra effects, but only on certain web browsers.
.GIF : These are for animated pictures and can even have transparent backgrounds.
.PCX : This is a good format that doesn't blur the sprites. This is also the format that is used by M.U.G.E.N.
.PSP, .MNG and .PSD : These have the benefit of saving many frames of animation as layers that can be later used in some animation programs like Animation Shop.
.BMP : A good format for saving single frames. The problem is that each file takes up A LOT of space.
.JPG and .JPEG : NO! They ruin sprites.

Example: If the frame rate is 60 and the sprite appears for 3 game ticks, you multiply 3 by 1.667 (100 divided by 60). Here's a formula for it:

Remember that many different factors will still alter the timing of a sprite on a webpage. However, making sure that the timing is accurate will keep people from complaining.