Seeing as I have just had to up the size of the promo image, I thought about how we could try and keep things running as quickly as possible. Google suggests serving images in modern formats such as WebP and Jpeg 2000. So we have now moved over from serving our compressed Jpeg promo images to compressed Jpeg 2000 promo images, I'm not sure if there will be compatibility issues but we will see how we get on.
Luckily you don't have to do a thing, although I highly recommend updating your promo image due to the new size. I have automatically converted all current images over to the Jpeg 2000 format.
If you do have any problems seeing any of the images please let me know!
Thanks,
-iDev