Yes FB is very strict about 20% test rule on the images. I typically do this..
1 - Find an awesome image form free website (here are some)
http://www.gratisography.com/
www.unsplash.com

2 - Add text or puzzle I want on this image
3 - Check the 20% rule using FB grid tool
https://www.facebook.com/ads/tools/text_overlay?_rdr

4 - Promote on FB

hope this helps :)

The Facebook grid tip is a good one, Canva is also a good (pretty inexpensive - images are free or $1 an image I believe) tool to grab an image you can edit / layer text on top of for things like social media posts / blog headers (they have "Facebook" specific sizing and its built to be editable by non-designers):

https://www.canva.com/

And there's a good list of similar tools here:

https://blog.bufferapp.com/tools-create-images-for-social-media

Great question, I would think that Facebook has algorithms and people on tasks to review this simultaneously. Your question regards more so "hacking" into their platform to just promote a picture. Don't know why you are trying to do this, but maybe find another way to promote the pic, such as tweeting it and then promoting that text on facebook or something.

This site has some great pictures that will grab attention on Facebook ads. I speak from experience. Check em out!

http://www.gratisography.com/