On this site, the standard image sizes are as follows…

• Images that span both the centre and right columns should be 650px x 186px.
• Images that span the centre column should be 410px x 186px (large). This is also the size of image required for the home page slideshow widget. You can hang pictures narrower than this to either side of the main column – but this is likely to look best if the width is no more than about 195px.
• Unboxed pictures in either the left or right column should be 210px wide and preferably 186px high (medium).
• WordPress will also create 150px x 150px thumbnails.