long-ish (~1400 chars) post about hashtags on Mastodon, and accessibility.
@Leftcel_Infilitrator while @guilevi has already answered your question about how to describe images, I'd like to add some information about hashtags. :)
Hashtags on Mastodon are primarily used for content discovery. Most servers/instances only allow you to search for toots by hashtags as search for any text in the message (often referred to as 'full-text search') is resource intensive.
So, if you want people to find your messages while searching for a specific topic, having the right hashtags in your message is often the only way. Not just for finding other people's messages, but also finding back an earlier status you posted yourself.
What can help people with screenreaders when it comes to hashtags, is to capitalise ever word in compound hashtags; also referred to as camel-casing. For instance, rather than #accessibilitytips, write #AccessibilityTips, as some (most?) screenreaders will use that as a word-boundary hint, and thus allow them to pronounce the hashtag as individual words.
Whether to use inline hashtags, replacing the actual word, or to list them all at the bottom of your post, is a personal preference from what I understood. If you use a lot of hashtags, I assume a summary at the end rather than inline is probably better, but since I don't use a screenreader myself, I'll defer to the expertise of those that actually do use them.