I posted a trip report for a hike in Vermont which had an appearance like the London WNBR, with 3 columns of thumbnail images and text to the right of each image. Unfortunately when you clicked on the thumbnails, you'd see the full-sized image, but then when you closed that image, the thumbnail would be replaced by the full-sized version and the page would become almost unreadable. I think this occurred because I was being lazy, and trying to use the same file for both the large version of the image and the thumbnail, as shown below, where the actual image is DSCF4709.jpg. The second reference to the file should instead call for a thumbnail file (T_DSCF4709.jpg or whatever) which I should have made available.
[url=https://dl.dropboxusercontent.com/u/28291527/Naturism/WNBR%20London%202016/DSCF4709.jpg]
[img width=200]https://dl.dropboxusercontent.com/u/28291527/Naturism/WNBR%20London%202016/DSCF4709.jpg[/img][/url]
My method is unkind to readers, anyway. What it does is load all the images at full size in the user's browser, but with an instruction "show this picture at a size of 200 pixels width". It's sent full-size to the user, and then if a request is made to see it full sized, it's sent again. If I had used proper thumbnails, the user would have less data to download, and it seems that the formatting stays correct after an image is viewed. I'll be doing it that way from now on.