Great! Stumbled upon Paul’s post by chance.
I was afraid that I had lost my pictures for good, but this trick allowed me to download the images and archive them on my HDD.
However, I must say that the late Andrew Waugh’s definition for high resolution in post #2, and what is commonly considered “high-res” don’t agree. This can’t be the reason for the glitch.
Examples:
It belongs to my old album:
http://www.jag-lovers.org/snaps/snap_view.php3?id=1419987798
The image is 980x1180 pixels and only 129 kB.
The problem repeats itself for almost all of my 19 albums, yet, I used to resize all images that I made into albums to the optimal size and optimized the files for web-viewing.