When designing graphics for the web make sure you have optimized your graphics before you publish them to the web. Large graphics will take a long time to download. For Adobe Photoshop 5.x & Above when you go to save your file go to 'save for web' under the 'file' drop down menu. You will able to optimize you image there.
.::Use the right image format
Using the right image format can have a severe difference in the quality of you image. Use JPG's (JPEG) for photos & scanned images and use gifs for icons, diagrams & other pieces of graphic art.
GIF(Graphics Interchange Format)
JPG(Joint Photographic Experts Group)
.::Cut your images up
If you cut large images up into smaller pieces your website will load more efficiently and have more
chance of the customer hanging around. Try to keep a single image below 200x200 pixels.
When creating transparent images design them in on the same colour background as your webpage. The antialiasing around the edges of your images can look quite ugly if it does not match the background.
Transparent image created on a white background
Transparent image created on a black background
.::Keep graphic sizes down
Big graphics can take a while to download. If a customer has to wait too long for your site to display they may lose interest