php widthxheight category, take also a look at the manual
width and height is in pixels, category is eg nature or sports, take a look at the description for all possible values.
NEW: this version uses better and prettier images, not just boring monochrome pictures.
Please take also a look at our other extensions at http://extensions.joomla.org/extensions/owner/extromedia
Take also a look at our new demo site at http://demo.extro-media.de