A couple of users in our Jive community have asked about the image size for the banner tile. I've had a look around the JC and i couldn't find the information. I played around with different sized images and the following works best for us, in terms of dimensions and quality of image. I did a test with an image sized much smaller, the dimensions worked well but the smaller size compromised the quality of image.
960px X 250px keeps everything that I cropped in the image itself, without zooming in so much. This image size looks great when you display just one image:
800px X 500px looks great when you display 2, 3, 4 or 5 images on your banner tile:
Note: When the images are loaded into the banner tile, as it's a fluid design, there will be some natural cropping/zooming from Jive. (see two images above, compare the man waving to the three images below it. The head at the bottom in three images is much more visible). As long as the images have enough space and any faces/text isn't too tight to the edges, it will work fine.
Hope this helps...