There are 3 steps involved in making your photograph look good on the Internet:
Selection: Select a photograph that is bright and colorful. Photographs in general tend to become darker when seen on the computer which is why nighttime photographs are not displayed very well. Select a photograph that is preferably taken outdoors, in the natural light. Also, if possible, select a close-up or a full view snap. Try not to choose a group photograph where you are only a small part of the whole photo. Lastly, choose the photo that brings out your features.
Enhancement: Once you have selected and scanned your photograph, you can further enhance it using image enhancing softwares like Picasa. Working with even simple functions like brightness, contrast, hue and saturation can make a big difference to your photo.
Optimization: Finally, make sure that you save your enhanced photograph as a .jpg or .jpeg file. These file formats are web-friendly and reduce the file size considerably. Having a small file size is important so that it loads quickly on the Internet.