You can't prove who you voted for today: that's a crucial part of the electoral system, not a minor feature. How are you thinking that you could you prove who you voted for?
(Keep in mind that you can always request a new ballot if you've "spoiled" your original one, so a photo showing your completed ballot could easily be faked. And my memory is that it's officially illegal to take photos in a polling place anyway, so if you tried to take a picture as your ballot was being fed into the machine, the poll workers would stop you.)
Crypto would allow voters to follow their vote through tabulation, in private, and see exactly how it's being counted.
It has only two downsides...
1. People could prove who they voted for after the fact.
2. People who don't understand technology may not understand how it's so secure.
...but the reality is that these issues exist in the current system as well.