@LoisBryan hi :)
A tip for ALT on mastodon : describe the image, in a few words or sentences :)

You can put your name in the ALT and a few other infos, but here it's used primarily to include visually impaired and Blind people. Repeating the post doesn't provide them any new info.

I'd write something like "a watercolour-y digital painting of a blue vase with pink flowers. Next to it is a pink cup of coffee with a wide brim and a little round handle on a pink saucer. The image is loose and dreamy, the colours soft and pastel".

A lot of people don't share images with no or inadequate ALT, to keep their profiles accessible. If you want more infos about exactly how to write alt text, there are guides around, and you can also use the tag Alt4Me. People browse it when they have time. You can then edit tour post to include the text as is, or modify it. Hope it helps :)