A small note that's rarely discussed outside of tumblr is that alt text isn't accessible to everyone. Many people who require image descriptions use programs that enlarge text, invert colours, change fonts, etc, and even with alt text now being visible, it's not always enhanced by their accessibility aids
On tumblr specifically, the four biggest mistakes I see are:
Writing descriptions in small text. This is not accessible, full stop. Someone who needs larger text won't be able to read the small text, defeating the purpose of the description in the first place
Putting a description under a readmore. This is also just straight up ableist no matter how you slice it. For one, if you deactivate or change your url, the readmore link breaks. That's right, if you decide you're done being lawbreaker-extraordinaire and decide to change your url to we-stay-silly, all your descriptions are now inaccessible! Secondly, forcing disabled people to jump through extra hoops to access what everyone else can is ableist. It's like if public buildings had their ramps around the backs of the buildings or only let people use elevators by asking staff to unlock them every time. Some people put descriptions under readmores to reduce post length, but as tumblr users have always been able to filter the long post tag and tumblr now automatically shortens long posts, there's really no excuse for this anymore
Putting the descriptions long after the image theure describing. Keep them right under the image so people who need them have context for whatever you say next
Not actually labelling their descriptions. Sometimes if people are transcribing, say, a tweet or a Facebook post, they'll just transcribe the text of the post below. Problem is, without labelling that these are descriptions, sometimes they're impossible to distinguish from commentary on a post. Just label them to make it clear what it is
