Resolution

Technically, photographs of the Internet have no resolution, they merely have a size. Pixels/inch, picas/inch is a printing/scanning term for translating images onto paper. 

Confused? So was I

Let me explain. A 72x72 pixel square image would have 5184 pixels in it. This is an absolute amount of information that does not change. If it were printed out with 1 inch sides, it would be 72 pixels per inch on paper. If it were printed out with 2 inch sides, it would be 36 pixels per inch. If it were printed with 0.5 inch sides it would be 144 pixels per inch. Although the size of the print outs differ, they all have the same number of pixels in them, just packed closer together or more spread out. 

72x72 image. If printed out at 72 dpi (dots per inch) it would occupy 1inch square	Same 72x72 image, stretched out to fit a 144x144 space. This is what the image would look like printed out over 2 inches square (ie 36 dpi).  It's a bit blurred because it's still the same 72x72 image stretched out to do the work of a 144x144 image.	144x144 image.  Since it's occupying a 144x144 space, it looks sharp.  If printed out at 72 dpi it would occupy a 2 inch square. If printed out at 36 dpi it would occupy a 4 inch square.

Time to throw away pixels

If you're taking photographs with a digital camera, the resolution doesn't come into play at all. You merely need to decide how much real estate on the webpage you want your image to take up, and then resample the image to that size. 

For example, using my digital S20 my pictures come in at 2048x1536 pixels.  This would roughly be two to three screenfuls for most average web surfers (your computer is probably set at 800x600 or 1024x768 resolution).  I wiill need to resample my image using an image editor to reduce the size by reducing the number of pixels in the image.  A good rule of thumb for "man-sized" figures is 300 to 400 pixels in width*.

From this	To this

This will involve throwing away pixels and information, so if you blow the picture up again to it's original size it will not be as sharp since information is lost.  It is always good to keep a backup of your images with the original number of pixels, in case you need to do close-ups of specific areas later.

Compression

On top of this, JPG (JPEG) files can decrease file sizes by compressing images using built-in algorithms, that make compromises with colour information and image clarity in order to reduce file sizes. The pixel size of the image remains the same, it will however lose more and more colour information as the image is compressed, and may exhibit artifiacts(strange squares, jagged edges) as pixels are rearranged into an optimum position. So you end up with a picture taking up less space, but not looking as good. 

If you are using Adobe Photoshop, the way to prepare your images is to choose File, Save for Web, and select a final image size, as well as decide how much compression you wish to apply. If you wish to process several files you can do so using File, Automate, Web Photo Gallery, which does much the same thing, but to all pictures in a directory. This output is saved in a seperate directory from the original, so you can experiment without fear of loss of your pictures. Paintshop Pro also provides similar functionality

Conclusion

Well, that's all. You're on your own.  Happy painting and snapping!