When I go to the edit screen for the orginal version (English) the feature image and other images are all using thumbnails (which they should).
However, the translation version edit screen shows the images in original size. hidden link hidden link
Not yet, second tier are still examining the problem to identify the cause which seems related to the version of the WPML API used in the reference site, but I don't know for sure.
I have asked if a workaround might be possible in the meantime.
Creating a new multilingual classifieds reference site either on discover-wp.com or installing locally, the translated post uses the correct thumbnail source image.
Given the simple workaround of the CSS rule above, they are dis-inclined to investigate further, but if you think they should, let me know.