Image Size Specifications
Images must be cropped or resized to fit one of the following standard image sizes. A useful, FREE Web based tool for image sizing and optimization is pixlr.com
Three Column Layout - Center Well
There are two widths for images used in pages with the three column layout.
If you're adding an image to the center well, it should be a maximum of 420 pixels wide. An optimal weight is 20-30k.
Example:

Three Column Layout - Right Column
If you're adding an image to the right column, it should be a maximum of 160 pixels wide. An optimal weight is 15-20k.
Example:

Two column layout
The maximum width of an image added to the content well on a page using the two column layout is 599 pixels. An optimal weight would be 40-50k.
Example:

Widgets
Widgets are used in the right column of the three column layout. Widgets should be a maximum of 160 pixels wide. Height is flexible, but the optimal weight is 15-20k.
Example:

Thumbnails
Thumbnail images are found on directory pages, normally using the Bulleted List Element They are 100 pixels wide by 60 pixels tall. The optimal weight for thumbnail images is 5-10k.
Example:

Blog thumbnails
The tiny images used for thumbnails in blog posts are 65 pixels wide by 45 pixels tall. They should weigh 5k or less.
Example:

Rounded corners
Creating rounded corners on your images is optional. Use a radius of 5 pixels.