Okay, thanks for clarifying what you had in mind. I eventually got it, I think:
I created a new product, uploaded a vertical image, let the theme create the thumbnails and published it.
Again the thumbnail displayed on the frontend was much bigger than it should.
After I figured out that this was the thumbnail named "-126x190" I copied this thumbnail file to my computer and opened it in Photoshop.
Then I opened a previously created thumbnail sized 188x190 px and copied it into the file from the Media Library and saved it.
Then I uploaded this file via FTP back to the Media Library, replacing the original thumbnail.
If you look at the frontend http://www.wp.fotowalther.ch/?product_cat=devins-suggestion you will see that again the vertical thumbnail
(DEV 1193) is neither displayed 126x190 (as its name suggests), nor at the size it really is (188x284 px) but 282x428 px.
So, in fact I was able to fool the theme and upload a custom sized thumbnail.
(The theme thinks the file "-126x190" contains still an image sized 126x190px, but in fact it contains an image sized 188x284)
However, IMHO the problem has shifted from an upload-problem or a resizing-problem to a display-problem.
(Therefore I will start a new thread to avoid confusion)
Please have a close look at http://www.wp.fotowalther.ch/?product_cat=devins-suggestion
Please remember: both thumbnails displayed are of the same size (188x284 and 284x188 px, respectively).
Do you see them all the same size? Is it only me to perceive DEV 1193 much bigger (almost double the size of DEV 1191)?
I checked on my wife's computer (different OS, different monitor). I see the same thing: DEV 1193 is much bigger than DEV 1191.
But both thumbnails are proven to be the same size!
I think this is a real problem!