What is a Full Resolution Thumbnail Without Full Resolution Image Link?
On the web, it's possible to display an image on a webpage downscaled without making clicking on it display the image in its full resolution. This is an usability bug.
Normally, the way it's done is that the full resolution image is processed on the server so that a low resolution version of it is created. This is the thumbnail. The thumbnail is smaller and highly compressed, so its file size is smaller, making downloading it faster, and allowing you to have more images displayed on screen without requiring as much RAM and CPU power. This thumbnails is made interactive, typically as a link, such that if you click on the thumbnail you'll view the original image on your browser.
However, it's possible to skip the thumbnailing step and embed the full resolution image directly on the webpage. In this case, the full image will be downloaded and then your PC has to do the job of downscaling it to fit the dimensions specified by the web page. In other words, instead of the thumbnail being generated once in the server, every single user who accesses the webpage has to download the full image and their computers will have to generate the thumbnail themselves. Clearly not idea, but not an usability bug.
The bug occurs if this "thumbnail" isn't interactive, such that there is no indicator that a higher resolution image exists. Normally, if you make the thumbnail into a link, the indication will be that when you hover the mouse cursor over the thumbnail: it will turn into a link cursor, indicating that you can click on it and, by convention, that this will open the full resolution image.
If the thumbnail isn't turned into a link, and image's dimensions is restricted to fit on the page or on a post's boundaries, then you'll see the image smaller than what it really is with no indication that you can view the full image.
In a typical web browser, you can view any image in its full resolution by right clicking on it and selecting Open image in new tab in the context menu. This will open the image's URL in a new tab, and since this tab only displays the image, no restriction will be placed on the image's dimensions by any webpage.