I've acountered additional problems.
No matter what I do I get a huge loss in image quality after image upload.
(again portrait format, but this time both in the preview AND when I click on it)
First: My upload was bigger than the preview image, so it shouldn't loose sharpness.
But that is not the case. The preview is more blurred although downscaled.
I am betting on a bad scaledown algorithm here because the blurring is inhomogenous
on some upload widths I tried**.
But that was only the less serious problem.
If I click on it to get a better view it gets even smaller and equally blurred.
I suppose that should never happen.
As I understand it when one clicks on the preview picture one is supposed to get the
original uploaded picture 1:1 often called "actual pixels view".
I tried widths of 700pix, 640pix, 1024pix, all with the same result.
(It was an exported vectorgraphics file where the blurring was especially good visible on sharp edges.
All of the versions had an aspect ratio of width*highth = 1024*1174)
Finally I tried to prevent the scaling with uploading exactly the shown size:
I right click-downloaded the picture preview and the "clicked view".
Both had the same size so I guess I can only access one of these two views.
I got a width of 428 pixels, scaled my picture on my pc down to this with (checked quality) and uploaded it. This still lead to blurring but most of it was in the center. The sides where sharper (** inhomogenous blurring as mentioned before)
Strange thing is that the autogenerated picture blows up way bigger to a width of 700 pixels when I click on it.
summing up there are two problems plus the one from before:
# scaledown for preview doesn't perform well
# the high quality (clicked) view doesn't show the original uploaded images actual pixels and is sometimes even smaller then the preview image
# high aspect ratio portrait pictures are blown up to fit the width