You need a program like pgotoshop, photo suite, or any any of a bunch of similar image manipulation programs.
Different files have different sizes and different ideal resolutions.
JPGS are usually used to store raw images, or more often to load on the internet as images. The file small is small. To keep a jpg looking good, take the size down to about six inches at 72 dpi. This generates a small image that still looks good on screen. If you printed it however you would see the pixels.
For printing you want to make sure the resolution is 300 dpi. Save these as TIFs.
PNGs are often used in other formats, such as the texture in 3D art. picts are similar, they are small enough to use in the 3D environment.
PSD and TIF files can have a lot of channels. One for red, one for green, one for blue, then an fourth channel to use for transparency. These two formats can also have layers, but that's a bit complicated for this discussion.
EPS files are for printing typefaces and logos. They are vector graphic, you can make them bigger without any pixels appearing.
You should get yourself a copy of Real World Photoshop. It's a great book for people interested in such things, and has charts of all this kind of information.