Images will load in the following cases:
- on initial page-load if on-screen, OR
- when they are outside the current window boundaries but an image edge is within thumbnail height of the upper or lower window edge and:
--- the scrollbar has changed position since the last calculation, OR
--- the page was redrawn due to window resize or portrait-landscape change.
You can watch the browser's network activity to check whether these rules are followed as designed.
Sometimes server response times will be slow enough that you can see the image loading, but most of the time that should be completed off-screen. If you see that an image never finishes loading, check the browser console. Perhaps there was a Javascript error that got in the way or perhaps the image resource was unavailable.
For long pages with high number of images, this lazy-loading of wiki images saves significant server resources.