When an image file is uploaded into a course, the Ally gauge only displays if it is 'drag and dropped' onto the course AND the activity name is not changed/edited.
If you use the File Picker to add an image, the Ally gauge will not display.
ALLY say that this is functioning as designed:
This is known and expected behaviour. It occurs because of the predictive nature of determining whether alt text is meaningful or not.
Note that the underlying issue is due to the file name and alt attribute, and not the drag / drop or file picker. We don't expect any problems with the file picker or drag / drop.