Problem 1: The logo does not show up in the marketplace listing
Problem 2: When I try to install it via openHAB binding installation page I just get this non-descriptive error:
No info in the openhab.log.
It installs fine for me on a recent 3.3.0-SNAPSHOT build.
Note that there is also an add-on cache in openHAB so it can take 15 minutes for URL changes in your post to take effect. To wipe this cache you can for instance disable/enable the Marketplace or restart openHAB.
The logo needs to be added to the forum post using drag & drop. AFAIK you cannot link to an image that is hosted on another site.