Just as the title says, is it allowed to add the logo of the corresponding game to tagwiki's? For example, I just edited the tagwiki roller-coaster-tycoon-3 and added the logo of the game at it.
If that is the only edit you are doing to a tag wiki/excerpt, it isn't substantial enough. There isn't any real reason to add these to the wikis, as generally these are just supposed to be a brief bit of text to tell people what the tag is about. Does adding the logo give any sort of extra information that the poster needs to understand the tag? Not really. I would then suggest avoiding adding them, as they don't add anything to the tag wiki that is really necessary to understand the tag.