There are two ways to add alt. text to images in announcements.
The First Way is Uploading Through Selecting Camera Icon
Select the Camera Icon
Select “My Computer”
Select the Image You Want to Upload
A Pop-Up Will Come Up Add the Alt. Text in the Box
Unless the image is decorative then select the box and you will not need to add any alt. text to the box.
Select “OK” Which Will Add the Image with the Alt. Text
The Second Way is Dragging the Image to the Announcement Box
Drag the Icon into the Announcement Box
Select the Image
Select the Three Lines
Type the Alt. Text Into the “Image Description” Box
Finally, You Post the Announcement the Same You Always Will
Another great resource would be this article about alt. text and what should be included when describing the image: How to create Alternative Text.