I had forgotten about this post here,  but yes i did some further testing in relationship to this post here as well https://developers.oxwall.com/forum/topic/32410  which does work to change the image. But then we are left with the question why does this happen.

So with regard to the user settings "guests can view the site".  There are times that we want certain parts of the site to be viewable by guests so many of us choose yes for this option and then block specific content as we like.  

This means that if you run the site this way with "guests can view the site" as yes, then you cannot share blog/forum/group/event and other internal site links without also going into user permissions and selecting guest on each of those items.  If you do not then you will get the Limited permissions display.  

However if you do that then guests will be allowed to view the full feature (ie forum) and you cant stop them if they know the url.  Now you can keep the button from appearing by editing the buttons but if they know the url then there is nothing you can do.

So its a choice, 

share links - share full features - share limited other site content

OR

dont share links - dont share full features - share limited other site content

This is something that needs to be addressed by oxwall one day i hope..