Upon image upload, regardless of method, the alt-text associated with that image is entered. That alt-text would stay with the image, as the default alt-text, when it is used on a page. You should still be able to edit the default alt-text content on a use-by-use (page-by-page) basis, but the default behavior would be that we could enter alt-text for an image once, when we upload it, and then that is the default alt-text for the image when it is used on a page.
If you want to change the default alt-text for an image, you should be able to do that in the Files -> images area.
If the image is uploaded in My Files, and alt-text is associated with the image, that alt-text would be the default alt-text anywhere the image is used throughout their Canvas account. In other words, for images that I use across courses, I should, again, only have to the alt-text once. Specific use of an image should allow for modification of the alt-text for each use, but I should not have to re-type the correct alt-text over and over again as I reuse an image, especially one in My Files.
Why: I use several images repeatedly in my modules. Each time I use an image, I need to replace the default alt-text of the file name with a meaningful name. Over and over, I am re-typing the same alt-text for a given image. From an accessibility perspective, is there any reason why we would want to be forced to do this?