Images

JPG

• 24-bit color, grayscale; lossy compression; interlacing
• Well suited for photos

PNG

• Portable Network Graphics
• Multiple color depths; lossless compression; interlacing; palette-based and alpha transparency; gamma correction; text annotations; a subset of mng
• Pronounced "ping"

• Alpha transparency support: Firefox, Mozilla, NS6, Opera 6, Safari, IE7
• Simple transparency support: IE 6
  
• No transparency support: NS4, IE <= 6 (inadequate support)
  

GIF

• 8-bit color; lossless compression; proprietary format; palette-based transparency; interlacing; animation
• Well suited for icons, diagrams, and user-interface elements
• Pronounced "jif"

SVG

• Scalable Vector Graphics
• Supports audio, video, and animation

MNG

• Multiple-image Network Graphics
• Animation of PNG and JPG images; supports transparent JPGs
  
• Pronounced "ming"
  

• Only supported by Konqueror

JNG

• JPEG Network Graphics
• Combines jpeg with alpha transparency; a subset of mng

Misc

• Interlaced GIFs are equivalent to progressive JPGs.