I'll only speak briefly but I'm pretty sure the consensus among most comic artist as well as writers who've been on the site for a while is that art theft on Tapas is simply a no go (plus I'm pretty sure it's mentioned within Tapas' own Creator Guidelines)
This bit
Is pretty interesting because sure a mindset of an indivudual will follow them no matter where the go but a person with sense will realize that if it's not approved or allowed you just won't do it. And sure there are the stubborn type who refuse and continue doing what they're doing (this case being art theft). To be honest I've browsed Novels/Comics that are fanfiction of popular franchises and it's pretty common to find stolen art there but I imagine after communication with one person or another they may or may not change it.
Even as a comic artist I can empathize with this but it's one of the reasons we try to share resources to help each other out (which is kind of the point of a community). There are folks who in some cases will offer to do covers for discounted prices or offer tips and tutorials to better polish ones folks already have.
Plus as was mentioned there's no harm in just asking an artist if you can use their art and if they say no you can try to find another. It's one of the reason's I've stuck with the Tapas community for so long is because for the most part people respect one anothers work. It's just common courtesy.
Edit: and Joanne has shared Tapas' clause in the TOS related to the matter