Below is a list of acceptable file formats for web graphics and photos to help you achieve the best results visually as well as reducing file size for your online community.

.jpg

JPEGs are designed to efficiently compress continuous colour or grayscale images, such as photographs or fine artwork. JPEGs do not work well on hard-edged images with areas of flat colour, these are better suited to GIFs or PNGs.

.gif and .png

GIFs and PNGs are used for flat-color images like logos, buttons, and graphic text, and are based on the number of colors contained in the image. Finding the least number of colours without compromising the quality of the image will yield the best results for keeping your file size down for the web.

When it comes to compressing files for web PNGs tend to compresses better than GIFs (in most cases). However, Internet Explorer 6 does not support PNG graphics while Internet Explorer 7, Mozilla Firefox, and most modern browsers do.

Use PNG files for creating transparent backgrounds that are continuous (uses alpha channels), not flat - for example if you have a logo with a large drop shadow that you want to blend gently into a background. GIFs also support transparencies however are better suited for flat transparency use, either a pixel exists or it doesn't - for example, a button that has straight solid edges.

.bmp

BMP files format, sometimes called bitmap, is used as the standard storage format in the Microsoft Windows environment. Although it is based on Windows internal bitmap data structures, it is supported by many non-Windows and non-PC applications.