See https://osmc.tv/corporate-and-legal/#trademarkinformation. There is a level of quality expected with OSMC, and unofficial images could prove problematic and give people a negative impression, particularly if they think they are in some way officially endorsed or supported.
There are also legal implications as most people want to include 'addons'
This would be a maintenance problem for us. We prefer to offer HOWTO guides in our forum at this time for those that want to expand their system.