"A picture is worth a thousand words" But, what thousand words did you have in mind? If you made a post that was 1000 words, but many of them were not related to the question/answer, that would be of low quality. You never know what someone will take from a photo - unless you direct them by having appropriate text. This stands no matter what quantity you wish to assign a picture.
My point there: They really are of no value (positive or negative) without appropriate explanatory text. So, in my view, they should not add any 'value' for the algorithm of which we speak.
Another problem: Without appropriate
alt values, images have exactly zero benefit for finding the question/answer, either via search engines or the internal search. If I had to guess, the quality filter is probably made with searchability in mind, to a large degree.
Also; on mobile, images are often of less use than on the full site. It can be difficult to determine what is being displayed on an image on a small screen, and while zooming/panning help (after loading the image in a new tab), it would be so much easier if the text of the question/answer described what was in the image first.
Images can be very helpful, but they should never be considered a vital part of the answer/question.