Optimizing Images for your Website
Use the Correct File Type
For illustrations and artwork – 8-bit PNG or GIF, reduce number of colors. Use PhotoShop and save images for the web – if you reduce the color palette from 256 to 32, the file size will be smaller…
For very detailed, colorful artwork or for photos, JPG and 24-bit PNG are typically used. While a 24-bit PNG results in superior image quality, it is a larger file size. Use JPG instead and adjust the quality setting so you can compress the image as much as possible while maintaining quality.
JPG has an option called "Progressive" mode. This option adds multiple copies of the image at lower resolution to make the image appear quickly on the screen, while progressively improving in quality. But it also increases the overall size of the image.
PNG also has a similar feature called "Interlaced". You may want to turn this feature off so that the full image downloads quicker.