From what I understand, fan materials are inappropriate for the wiki's article spaces unless there are recognized references to them in official materials, and even then those are usually strictly a matter of trivia. Some easter eggs referencing fan materials may be a matter of dispute, but some (like those referencing Sophie Campbell's TMNT fanfiction) are unmistakable and have been mentioned, as in TMNT #89.
Otherwise, there may be a certain leeway allowed (or tolerated) when it comes to fan materials on user pages, blog posts, discussion forums, etc., as these are effectively social media environments where fan expression is ordinary and expected. I link to my off-site TMNT fan music on my user page, and on rare occasion I've written improvised mini-screenplays in blog posts or forum threads, just for fun. Talk reaction images also occasionally materialize, and even The S has been known to upload images for this purpose.
What Turtlepedia is not is a primary archive site for fan art and fan fiction. Other sites are more appropriate for this, and better set up for it, too. But it can't hurt to talk with other fans and toy with concepts. The forum thread I linked at the beginning of this comment represents something that is perhaps near the upper limit of what could be allowed here, the nebulously-sourced artwork notwithstanding, because it can be interpreted as the sharing of ideas with other fans. Years back, another user once shared a draft screenplay with me seeking my opinion, and there seems to be no real harm in this, either. Things like that. No one seems to pretend that, when they post their stuff socially, that those pages become dedicated host URLs for their fan art or fan fiction.
But The S is the main arbiter of what is and is not allowed on the wiki, so when in doubt, defer to his judgment.
Some background: One of the reasons fan materials were removed from the wiki was because The S accumulated a number of complaints from wiki visitors. There was a debate on what was appropriate for inclusion in the wiki's articles. Some fan works have been very notable, had a large number of readers, and been co-created or read by official creators, including Teenage Mutant Ninja Turtles: Secrets of the Ooze (by Sophie Campbell), Casey Jones: The Movie (to which Robbie Rist lent his voice acting), and the various publications by Illusion Studios (some contributed to by the likes of Kevin Eastman and Jim Lawson). Front and center in the dispute was Mutant Ninja Turtles Gaiden, which has the distinction of being one of the (if not the) most widely-read and influential works of TMNT fan fiction in existence, having published for over a decade with hundreds of pages and counting, and formerly had an entire section of articles here on Turtlepedia. It has been high-profile enough that there is still serious question of whether some of its scenes were referenced as easter eggs in later officially-published works.
By Sophie's own word, official creators can and do read this stuff. However, MNTG's status as fan fiction was still enough to attract complaints from fans who came here expecting only articles about officially-licensed TMNT materials. Eventually, The S made a decision removing fan material articles from the wiki, no matter how high-profile they are. Most were migrated to the newly-created Turtlepedia Fanon Wiki, while Mutant Ninja Turtles Gaiden (which had by far the most articles) was migrated to the Mutant Ninja Turtles Gaiden Wiki.
(It seems I can never just give the gist of my thoughts on a given matter. Everything seems to have to morph into an essay. Not that this is always bad...)
Your contributions to the Rise of the TMNT image galleries are greatly appreciated, but please try to avoid uploading similar images/screenshots. I have been going through a lot of the galleries and noticing the exact same screenshot from a episode, but cropped differently.
Hi, while your contributions to adding pictures to galleries is greatly appreciated, please avoid adding artwork such as this into these galleries. Some of these are property to the artist, and I know some of them do not appreciate it when someone reposts it online.