You can add #AltText in both #Lemmy and #Kbin by writing the image description between the brackets
! [Put your alt text here] (url)
Also, when you upload images from your pc on #kbin there's a text box below (with the words "Imagine alternative text" above it) where you can put your image description.
Another thing, when writing tags with several words in it, always write the first letter of each word in Uppercase #/LikeThis, otherwise most screen readers/text-to-speech apps can't detect them.
Also, I'm begging you, avoid special fonts as they cannot be detected by screen readers and TTS software.
Most of the time the letters are actually phonetic or mathematical symbols and the software interpret them as such.