They are usually created in an illustration or drawing program. On the Web they are usually best saved as GIF files, rather than as JPEGs, since GIF tends to be more efficient for this type of image. Anti-aliasing gives the graphics on some web sites that slightly blurry look, but it also removes jagged edges, making curved lines look more curved. Some programs can anti-alias graphics for you if you have them saved in the GIF format. GIFs can also be used to make non-rectangular images by using the Transparency program.
Pictures change colors pixel by pixel like in photographs.
They are usually created by scanning or using a digital camera. JPEG gives smaller files and better quality for pictures, but some older browsers can only read GIFs. As you develop WWW pages, it is a good idea to view them in many possible environments, so it is a good idea to look at them on a Windows Netscape, on a Windows Internet Explorer, on a Macintosh Netscape, etc. You may be surprised at the differences. Technology, graphic design, and good writing skills all come together in a successful web site.