1:  Resolution
72 dpi is a web standard resolution
300 minimum dpi is required for all other media reproduction to avoid an image that appears awfully sub-standard.

2:  Length and breadth in pixels
The width is the first given number under these size examples . . .
167 x 250 pixels

400 x 288 pixels



600 x 309 pixels

800 x 162 pixels

A friendly reminder that there are NO FREE IMAGES on this site. Details of the image use license can be found here