Just to understand the ask... what are you asking them exactly enter into the alt tag field? Easily understanding what the image means typically involves describing the contents of the image. For example...
A caption might be "A Bored Cat". Now... if a user uploads an image, how is IPS going to know what the image is about for it to apply a proper caption.
When a user adds an image, there is already a spot for them to describe the image... that's what the image title is for.
Now... if they just added "User uploaded image" as the description, it puts something in the field, but it defeats the intended purpose of the field, which is to actually DESCRIBE the image. Just saying it's an uploaded image does not help an accessible user understand what is inside of the image.