http://www.purl.org/tcl/home/man/tcl8.4/TkCmd/image.htm
Someone want to mention the Img extension and why it is so useful?
See also Buttons with image and text - Images With Transparency and Plain Images - Reading GIF image dimensions - Reading JPEG image dimensions - Reading PNG image dimensions - Tk image Dos and Don'ts -
Tk syntax help - Arts and crafts of Tcl-Tk programming - Category Command - Category Graphics