Here's the img element of the post. Note that the alt attribute isn't *missing* but is instead *an empty string*. Without this empty string, the SR will announce the file name (or maybe just "image") which provide no value. Bluesky supplying a null value hides the image from assistive tech.

When image description would be meaningless or redundant, you should NOT supply any alt text in a bluesky post. Shoehorning in meaningless information is obnoxious, not helpful.