Search mock ups in the search bar at the top, there are lots of posts about this that will answer your question.

@Alice Taylor they are psd files but can be used in canva, photopea as mentioned by Camilo and a few other editing softwares, if you are unsure juts try open it in what ever you use and see.

@Jake Kittow They will be on automatically, I have kept them on as i think they look good, its personal preference id say.

@Peter Gordon Im curious why do you opt for that plan? I used the basic plan at $10 and found it was more that enough even with the limited 200 generations which allowed me to make far more than 200 with having 4 generations per prompt and a high difference in each design.

@Peter Gordon Fair, I dont think many people are working at that pace, the basic plan is more than enough for most I think, at least for just starting out I think youd be beter off geting the cheaper plan to try it out before going all out.

I have personally offered all 3 and every one of my sales has gone for matte so thinking of taking the choice away and keeping as just matte.

Im not 100% sure if it would or not, a quick google tells me techically yes you can but you need to adhear to a strict set of rules like not using the logo, not using licenced lego characters, the actual mini fig is trademarked and the art can not look like an official, endorsed LEGO product. It should be clear that your work is fan art, not a licensed product. That being said, I stay away from anything remotely copyrighted or to do with other companies, not worth the risk imo.