You can upload the alternative text in Site Builder for an image by knowing the filename of the image.   Most browsers have a Open Image in new tab option, when you hover and right click on the image. This will give you the file name such as "My_profile_image.jpg",  you can then do the following:

• Logged in as a Site Admin go to Admin -> Content -> Files
• Search for the file name using either a partial or the full name.
• Find the image in the list and click Edit
• Add the alt text
• Save the image
• Go back to the  Profile page or any page where the Alt text was missing, you’ll see the alt text as updated.