For my comic, my team and I just used some artwork that we did in development and polished it up for our Thank You images. I do notice that multiple characters seem pretty popular. 
This is our MC Markesha, with her technical engineer Audrey.
And then Markesha again with her dad.
Our pages take so long to produce, that using work we already had with minimal effort really paid off. It didn't take long to make, and people really like them. Stick to your main character or cast. Use characters who are relevant to the story even from the first go, and I think you'll be fine.