When opening emails which contain images, often the images don't download quickly. All I get are blank boxes with a tiny square in the centre. (Right click gives options, including Open Image in New Window.)
If several images in the message, sometimes some will appear while others do not. They will download eventually, usually if I close and then re-open the email.
I occasionally get the same blank boxes when trying to download images from other sources besides email. For example, when I tried to download an offer voucher from a website, the barcode etc would not appear - all I got was that blank box with a tiny square in the centre.