Hi Samuel,
They are very basic 30-100kb files, small sizes, done in Photoshop at 72.
I am really confident I am not making any glaring mistakes on my end, as I am working in 8-10 different WordPress sites as a living all day and night!
Kmessinger,
When I run the console and do the image uploads on any other wordpress site, the async-upload.php error doesn't show up!