Server load issues/lag

Slightly strange one- I use the HTTPS version of the site at work, and post server issue I now frequently have no images displaying in posts- and no little broken image picture/red cross to show that the image cannot/will not display.
If it is something that I want to see then I have to load the page in my iPhone, and the image will appear.

I have the same thing happening here. Not too often - but can't see why it happens to some posts and not others....