Well, that's a fine string of rationalizations we have going there.

- Did you use someone else's original art? Yes? They get credit.
- Did you use someone else's original mesh in your 3D model? Yes? If they didn't give a specific license that the model could be used in any and all works, commercial and non-commercial, then they get credit; if the mesh was purchased with such a non-exclusive license, then credit isn't necessary.
- Did you use screen caps from an episode? Technically, yes, you should credit the original source, but given that this is the overwhelming origin of most art on this and most other Trek BBSes, it's pretty much become a given that these known and legitimate sources go uncredited. We tend to be a bit more lax about those, but don't let that lull you into thinking that, just because you've seen someone else's art "everywhere," that it falls into the same category.
- Did you cut-and-paste original art, whether from a commercial or a fan source, to create the majority of your art? Yes? The original source gets credited.
This keeps coming up, and it's just not as complicated as people try to make it. If you did not create the original source of your artwork, you need to credit the person or persons who
did, and, yes, that especially applies to other fans' artwork or works that are not produced under license by the copyright holders. It's not only appropriate, it's simply common courtesy to your fellow fans - even those fans who may have gotten paid, legitimately, for their work. It's really
not about copyrights, because in many ways, most of the artwork that is not based solely upon the creator's pen, paintbrush, pencil or mouse is in violation of copyright in some way, and it doesn't matter whether you are making money from it or not.
If you build a model (real or virtual) based upon someone's design (like one of MJ's ship designs), you're not actually using their art - you don't need to credit it. If you paint a picture based upon someone's design, you don't need to credit it. If you paint a picture based upon someone's
art, however (say you do a
Trek picture that clearly is derivative of another artist's work, like a painting by Ralph McQuarrie ... or da Vinci ... or
Forbin ), credit it. "Fair use" is for things like commentary, or parody, or public review - it's not for things like creating your own pictures out of the pieces of others' original work.
What it comes down to is this: if it's in the board rules, do it. It doesn't matter whether you agree with it, it doesn't matter how you wish to interpret it or to circumlocute it - just do it. You agreed to abide by the board's rules when you joined. And if you see what you believe is a violation of the board rules, notify the moderator! That's why we have the button, so these things don't get out of hand - as you can see, non-mods who try to moderate anyway usually end up in the midst of unnecessary friction.