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 “Upload”
Select the Image You Want to Upload
Select “Add”
A Pop-Up Will Come Up Add the Alt. Text in the Box
If 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 Alt. Text to the Image
Finally, post the announcement like you usually do by selecting the "Publish" button.
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 at the bottom of the image.
Type the Alt. Text Into the “Image Description” Box
Select “OK”
Finally, post the announcement like you usually do by selecting the "Publish" button.
Helpful Resources
Another great resource would be this article about alt. text and what should be included when describing the image: How to create Alternative Text.