accessible math

I am a web producer for The College Board. The College Board is a not-for-profit membership association whose mission is to connect students to college success and opportunity. We are always trying to improve the accessibility of our web site and have recently started using Math Latex to generate math images for the sample PSAT questions we post on our site.
 
My question to you is, does it make any difference if all the values and variables and symbols are images as long as they are alt-tagged? For instance, there may be cases where we would use Math Latex to generate an image for "-2", so that we can alt-tag it as "negative 2" and then in that context we might also use Math Latex to generate an image for "2", so we can provide visual consistency for other users (since of course the numbers that are images LOOK different from the numbers that are not).
 
In your opinion, should we avoid using images for math equations unless we absolutely need to, or is it okay as long as everything is alt-tagged?
 
I thank you for your help!