[zamadatix]

In Chromium based browsers the quickest method I've found is "right click -> Inspect" the image then click the sources tab in the dev tools window. From here you can drag or save the image shown without issue. My guess as to why this works is the sources view seems to pull from the loaded content of the page rather than fetch the content based on the lack of packets trying this with a packet capture running.

[nine_k]

In Firefox, beside that, you can press Ctrl + I, open the "Media" tab, and pick any of the graphics that were already downloaded to display the page. Then you can save the picture(s) you're interested in. I suppose the source of it is the local cache.

Does not work in this particular case, of course, because the whole image is not yet in the cache.

[andersonvom]

It also works in this case, too: at some point the connection does close (if it doesn't, just hit escape) and you can save the image as usual, now from the cache.

[Kuinox]

Right click copy is faster.

[nine_k]

It copies bits shown on the screen, so it loses e.g. EXIF data, if any, the original color space, etc.