PureRef
Used by 3 peoplepureref.com ↗

“One of my necessities is Pureref, which you can drop or copy any image into and then move them around as needed.”
Keezy Young Interview, Cartoonist · via Uses This 
“PureRef is highly recommended to set up a speadsheet of references.”
ExitMothership Interview, Illustrator, 2D artist · via Uses This