Just to add, here's a slide from my last Moodle Prof Dev regarding Atto's Accessibility Checker and Screen Reader Helper.
Below is an image of a text box with two different messages below it. The messages are pop ups that appear when you press the Screen Reader Helper button. Basically, it will show you if you have provided a description for the images in the text box or not.
The first message displays what the reader would read out if a description is provided ("This is a description")
The second message had no description provided, so according to this button, a screen reader would read the file path as the description (http://school...)
As for the other button, you'll get a notice regarding color choices for people with visual impairments.