Have you posted a photo on Mastodon and the preview image hides something important?
If you're using the web version of Masto, you can alter the focus of the preview image when posting photos. Just click "Edit" on the image and move the circle around. When you've finished click "Apply".
(And while you're editing, do remember to add an image description for blind people!)
The two photos below are identical, but one has the focus set to the right and one to the left.
Yes, questions always welcome 
I don't use screenreader software myself but I think it scans the code of the page so that it doesn't have to hover over images.
I'm not sure what you mean by a message button? You can reply to posts by clicking the arrow or the speech bubbles below the post, you don't need to use the "...".
To reply privately to someone, @ them and set the privacy to "mentioned people only". You can change privacy at the bottom of a post, the privacy button will look like a globe 🌐 or a lock 🔒 or an @ sign depending on what its current setting is.
The official app doesn't have a message button (yet) but the website version does, it's called "direct messages" or "conversations" but it does exactly the same thing as @-ing someone with "mentioned people only".
Ahhh ok, yeah, that's true, you have to click "...". I don't know why they designed the interface like that.
You can also just click "..." below someone's post to message them.
Sorry, I should have said...
On the official app it's speech button plus set privacy to only people mentioned.
If you're on the website, it's "..." and then "message".
The official app is on phones and tablets, the website is on anything.
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Spike Smolders (she/they)