Resizing and compression are applied to dropped images larger than 500x300 in both dimensions. Here we study the compression artifacts for a test image from Wikimedia Commons. image 
.3 21831
.1 10335
.7 42151
.5 30319
1.0 375507
.9 81767
Each caption shows the requested image quality and the resulting data url size in bytes.
See 'Setting image quality with jpegs' in the Mozilla Canvas toDataURL API documentation. page 