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The best way to post an image on Mastodon:
1. Write your post and attach the image by clicking on the paperclip 
2. Don't press Toot yet! Instead, click on "Edit" in the corner of the image.
3. Move the circle so that the 16:9 preview looks nice. This is especially important on images that are portrait or square orientations.
4. Add a text description in the box, so that people browsing in audio will know what is in the image. (A lot of blind people browse the internet in audio through screen readers. Image text also gives you a chance to add details beyond your character limit.)
5. When you've done all that, click "Apply" and then "Toot!"
6. You can read image descriptions by hovering the mouse over the image after posting.
@feditips
I like to do all that in #Pixelfed, then boost the post on my #Mastodon and #Friendica accounts. #Fediverse FTW
@feditips
How do I add an image description AFTER I've already posted? I didn't find out about image descriptions until I had already made a few posts, and I want to go back and add them, but I don't see an option to do so in the edit menu.