Correct, they can't.
Facebook might use the AI to describe some pics, bu otherwise it's either asking for help from a sightee, or depending on what you can, zoom in and squint.

Crossposting from my answer to your asking about the ad in /r/funny:

So, much like how there are varying degrees of deafness, there are varying degrees of blindness. Because people can lose their sight partially, the ad may be visible for some people who have partial vision. The ad for /r/blind is about as high-contrast as possible with big blocky letters and three snoos (reddit mascot logo aliens). (For those who are exclusively using screen readers, the ad text reads "DID YOU KNOW REDDIT HAS BLIND USERS TOO? JOIN US AT /R/BLIND" and the snoos represent the blind community as simply as possible by having one with presumed normal vision, one with a single eyepatch, and one with light blocking glasses, a guide dog, and a cane.) Sure, someone who cannot see at all won't see the ad, but the ad is actually ideal for those with partial vision, even if it needs to be magnified it still gets the message across. For some totally blind redditors, coming across this very post might help them find /r/blind, so by posting the ad and asking about it in these comments you have actually helped them find that subreddit if they would benefit from it, so thanks for staying curious.

The screen reader JAWS has a feature called $1 that can try to read the text within an image. It won't tell the user there are snoos but it might successfully read the message.

It would be better if all the ad images had appropriate alt text.

You are correct, they're unable to. However, I'm a mod over on /r/descriptionplease, a sub dedicated to sighted people describing pics/videos for the blind.

TapTapSee
https://play.google.com/store/apps/details?id=com.msearcher.taptapsee.android&hl=en