Meanwhile I have followed up on the issue and consider it solved - largely.
According to my impression, what remains to be clarified - apart from the fact that Firefox at times has problems to open such a file (see attachment) - would be this. Permit me to reword the matter, even if this may lead to a few repetitions.
So I happened to bump into an effect so far unknown to me: 'images whose size automatically adapts to the window'. See example:
Thus it is an overlong image filling the frame, but is too small to read it. Clicking on it, it (rather a scrollable excerpt of it) appears in a reasonable reading size.
I found that link too complicated, so I changed that to
<a href="bst.jpg" target="_blank">hier</a>,
then - to test it - moved it into the website folder: This way I reached the same result like there.
Then I made another overlong jpg file, although with different measures. There, it also worked - on principle, but due to its different size, the result was no longer satisfying. After having resized it to the dimensions found in the first file (I had qualms doing that), the result was practicably usable again. See
http://pro-mobile-internet.net/Talk_4G.jpg
But it does not fill the window, length-wise.
So the resizing part appears to be a little insecure. What I would like to achieve is, being able to always master this situation - overlong image files - reliably. A clarifying explanation might be all I need.