The cover photo system is designed to be intuitive and will resize depending on what row your content board sits. In general, the following dimensions will apply:
1st row: 938 x 220 pixels
2nd row: 458 x 220 pixels
3rd row: 298 x 220 pixels
4th and below: 218 x 220 pixels
In Addition, the cover image is also used when viewing an individual board. The cover image in this case is cropped to 600x600 pixels, and width and height are adjusted in the browser to fit the background. The height of the final image will vary depending on how much text you enter as the board description.