Assuming that the album covers that you're asking about are FB Timeline covers, the image dimensions are 851px by 315px according to information on this page. The same information can be found at this Time Techland article.
Facebook has posted some information as well on this help page. The detailed specs include:
Cover images must be at least 399 pixels wide and may not contain:
- Price or purchase information, such as "40% off" or "Download it at our website"
- Contact information, such as web address, email, mailing address or other information intended for your Page's About section
- References to user interface elements, such as Like or Share, or any other Facebook site features
- Calls to action, such as "Get it now" or "Tell your friends"
To get the best quality image and fastest load times for your Page,
upload an sRGB JPG file that's 851 pixels wide, 315 pixels tall and less than 100 kilobytes.
If you're looking for the size of pictures in the photo album, this site provides this spec:
Pictures displayed in Facebook
This is the maximum size of the image as shown in slide show or album.
- Width: 960px
- Height: 720px
The photo album cover would be governed by the same limitations, since from my understanding the cover image is just one of the images in the album.