Thanks for your question
You can add alt text to image by app Paint , photoshop , photoscape on PC then upload them to you also add text in description widget
if you can’t config, please let me know name your theme ? & you could explain steps by steps to replicate the problem and provide us some screenshots , i will help you check and let you know solution.
Thanks and have a nice day!
Answer for How to add alt text to image widget?