I know why, it's because they have logos on it, which means it's a watermark type, such as YouTube symbol, only counts for fun, but others like Twitter, TikTok, Fake iMessage.com, 9gag.com, and others count as watermark. If you found those from the internet, you have to use a different template or it will not be featured in the fun stream, but repost stream counts for watermark type. If photos doesn't have a watermark, you are likely to post it in the fun stream unless you add a text and/or template also to post it. You have to be careful posting those if you are likely to follow the posting rules and the TOS, which is part of it.