Using your favorite image editor, follow these image size guidelines for the perfect fit.

VGA Image Size Guidelines

The entire VGA image size is 640x588. Keep in mind that some of that image is used for the landscape area of the theme. When your theme is in portrait mode, only 480 of the 640 pixel width is visible. In landscape mode only 376 of the 480 pixel height is visible.

Entire VGA image: 640 x 588
Portrait area: 480 x 588 (including command bar)
Landscape area: 640 x 376

SE Image Size Guidelines

The entire SE image size is 320 x 294. Keep in mind that some of that image is used for the landscape area of the theme. When your theme is in portrait mode, only 240 of the 320 pixel width is visible. In landscape mode only 188 of the 294 pixel height is visible.

Entire SE image: 320 x 294
Portrait area: 240 x 294 (including command bar)
Landscape area: 320 x 188

Standard Image Size Guidelines

The entire standard theme area is 240 x 293 (including command bar). The visible height of the main area is 268 pixels. The maximum visible area of the start menu is 190 x 290.  The actual size of the menu depends on the number of entries and the length of the longest item name.

The overlay above shows you the amount of the image displayed in both Portrait and Landscape orientations

This image shows you why the image should be tall enough to extend into the command bar.