This is the current design:
Where should I position the left-top element so this UI is more balanced?
I have two suggestions, depending on the aim of this product:
1) Can the image thumbnail be bigger? I would make the thumbnail roughly the same height as the QR Code and incorporate @John feedback.
2) Is that QR code simply for that one thumbnail? or are there going to be more thumbnails per QR code? If the latter, I would create 5 more gray boxes the same size as the original thumbnail and lay them out on a nice grid, so you have a 2 rows of 3 boxes.