First of all, why not put a picture on one side & a word on the other & teach both at the same time? It is a much better use of your time, you'll get a much earlier start & if you try to teach the word with flashcards later. If you do the same ones later, it is possible that your little one may not find it as exciting.
When you say that you are working on naming, do you mean that you wanting your 20 month old to respond with the name or do you plan to just give input? Our approach was just to input only, so that we were never pressuring our daughter. Only very recently, at age 4.5 have we ever asked K to read to us. Before, she just read to us when she felt like it.
Yes, fast flashing works for photos. We have done this for encylopedic knowledge cards - blog post to follow on this soon.
I've loaned my How to Teach Your Baby out right now, but I'll try to remember what I can about card sizes. I'm sure at that age we were already using a 4 X 5 inch card, because I remember my 2 youngest girls reading them together just before K's 2nd birthday. Usually, I used Little Reader's print function set to put a word & pic on each page, folded them in 1/2 & laminated. Do you have any idea how well your child can see? If in doubt, go larger because it is not uncommon for kids with Down syndrome to have vision problems.
It's wonderful that you are starting with familiar people & animals. This is always a great way to keep kids interested in learning.
I hope this helps. Please feel free to ask any other questions you have.