You can @ Lemmy communities and it will create posts there (not sure how well images federate)
Which instance you signed up only affects your “Local” feed (no idea if PixelFed has one, haven’t checked). If you’re posting primarily about your miniatures it may make sense to move to a specialized instance rather than a generic one like .social.
PixelFed and Masto federate really well so all you have to do is follow and get followed by people on the tabletop focused instances (which you already may be without realizing) for your things to end up in those communities.
There is also a.gup.pe though I have no clue how it intersects with Lemmy.