My approach was to create a site setting with my module that allows to pick a thumbnail from an asset for each site. Then, that miniature is used to render some of the block layouts that my module adds, like a site grid. This worked very well for the case since you can pick the image freely and make sure that it fits in the overall appearance. My client might release the code during this year, I’ll let you know when they do. Meanwhile, feel free to contact me at firstname.lastname@example.org if you need help with that or want details about the thumbnail solution.
Thanks for the offer, I will keep it in mind!
At the moment I’m going to keep investigating how I can pull up images from the sites image_set as it will give me the option to display a random image on load or similar novelties (performance permitting).
Depending on how long our upgrade to 1.3 takes that or your solution above may also help cover this area for me.