When designing your InDesign pages, you may use any image format you like on background layers.
When using non background layers, please ensure you are using one of the following image file types:
There are no specific sizes we recommend. However, the larger the file sizes the slower the upload and download will be.
For Image best practices we found this interesting article... Click Here for more information.
Please Note: Using oversized images with really high resolution can cause rendering issues on some devices, due to the devices having a lower memory, (iPad 1, 2 & iPad Mini).
Please see Retina Images for more information