
Here are a couple of tips and suggested links for optimizing images for web use. Images that take a long time to load can frustrate your users, so just taking the time to optimize your images will be highly beneficial.
1. Use the correct file format for the job. To find out whether to use JPEG, GIF or PNG, read below.
2. Set the dpi (dots per inch) to 72dpi. This is the optimum dpi for internet use.
3. Create the size image that you need – don’t use a large image and then re-size it on the web page. A re-sized image on the page will look pixelated and fuzzy and will still be the same file size. If you need a thumbnail to open up a larger image, create the thumbnail separately.
JPEG, GIF or PNG?
JPEG: If you are saving a photograph, or an image with a photograph in, a JPEG will be fine.
GIF: These are good for graphics and fonts as they are smaller in file size. You can also use transparency in certain circumstances with a GIF, for example, you might have a logo you want to place on a patterned background.
Note: Don’t forget to set a matt colour close to the colour of the background you will be putting the image on, or you will end up with pixelated white edges on your image
PNG: Has better compression than GIF, and you have the benefit of having more options with transparency, for example, you can set a drop shadow on an image with a transparent background and use it on a website.
Note: Not all browsers support transparent PNG backgrounds, but you can use fixes:
Twin Helix: PNG Fix
For more information on PNG, GIF and JPEG check the W3C schools tips on PNG, JPEG and GIF.
Further reading
More detailed information on optomising images for the web:
http://www.ehow.com/how_6236_optimize-picture-web.html
A very good tutorial for exporting images from Photoshop (also applies to Fireworks, etc)
http://inobscuro.com/tutorials/read/35/
A short but informative tutorial on GIFs and JPEGs with image demonstrations.
http://www.aksi.net/optimizing-images.htm