{"p":"","h":{"iv":"ROXSYW+cfvEbFHu5","at":"ocxplSQjdRC3tXEtB/9/wg=="}}
I figured it out! Long press on the photo, then the alt text appears at the top of the screen where there is a 'more' link to expand to read the whole text. At least this is what happens using the webapp. Using Fedilab, just click the photo to see it zoomed, then again to see the alt-text.