@Katiame https://youtu.be/Pg0rtrUOoJY?si=88pTO6Xgomz36dms&t=70 shows you how to add alt text when posting. To learn how to edit a post you've already made, see https://fedi.tips/how-do-i-edit-posts-in-mastodon/ for a screenshot (basically, you find the three-dots menu under your post, and one of the items in that menu is "Edit"). Editing a toot will send you to the same interface as composing a new toot, so at that point, follow explanation in the video above. If the video isn't helpful for you, let me know and I'll share a sequence of screenshots. @FediTips would it make sense to include a link to a video like this, to show visually how to add alt text to the Fedi.tips website?