I've created the database of buttons by splitting, cropping, masking and resizing images of buttons sourced from various places. All source images are licensed to permit sharing and adaption and I've attributed the creators of these images under attributions below.
Attributions
I've sourced the images of buttons from the following creators:
Creator: https://www.flickr.com/people/93410621@N05 License: https://creativecommons.org/licenses/by-nc-sa/2.0/
Creator: https://www.flickr.com/people/106074308@N06/ License: https://creativecommons.org/licenses/by/2.0/
Creator: https://www.flickr.com/people/volvob12b/ License: https://creativecommons.org/publicdomain/zero/1.0/
Creator: https://www.flickr.com/people/twenty_questions License: https://creativecommons.org/licenses/by-nc/2.0/
Creator: https://www.flickr.com/people/markmorgantrinidad License: https://creativecommons.org/licenses/by/2.0/
Creator: https://www.flickr.com/people/presley_m/ License: https://creativecommons.org/licenses/by-nc-sa/2.0/
Creator: https://www.flickr.com/people/130331218@N03/ License: https://creativecommons.org/licenses/by-nc-sa/2.0/
Creator: https://www.flickr.com/people/mag3737/ License: https://creativecommons.org/licenses/by-nc-sa/2.0/
Creator: https://www.flickr.com/people/deanhochman/ License: https://creativecommons.org/licenses/by/2.0/
Creator: https://www.flickr.com/people/littlelixie/ License: https://creativecommons.org/licenses/by-nc/2.0/
Creator: https://www.flickr.com/people/obd-design License: https://creativecommons.org/licenses/by-nc-sa/2.0/
Creator: https://www.pexels.com/ License: https://creativecommons.org/publicdomain/zero/1.0/
Creator: https://pikrepo.com/ License: https://creativecommons.org/publicdomain/zero/1.0/
Creator: https://pixabay.com/ License: https://creativecommons.org/publicdomain/zero/1.0/
Creator: https://pixbay.com/ License: https://creativecommons.org/publicdomain/zero/1.0/
Creator: https://pixnio.com/ License: https://creativecommons.org/publicdomain/zero/1.0/
Creator: http://www.readyelements.com/ License: https://creativecommons.org/publicdomain/zero/1.0/
Creator: https://www.flickr.com/people/salvagenation License: https://creativecommons.org/licenses/by-nc-sa/2.0/
Creator: https://www.flickr.com/people/14903992@N08 License: https://creativecommons.org/licenses/by-nc/2.0/
Creator: https://www.flickr.com/people/shellysblogger/ License: https://creativecommons.org/licenses/by-nc-sa/2.0/
Creator: https://www.flickr.com/people/thevintagesailor/ License: https://creativecommons.org/licenses/by-nc/2.0/
Creator: https://www.flickr.com/people/23882161@N03/ License: https://creativecommons.org/licenses/by-nc/2.0/
Creator: https://www.flickr.com/people/welshkaren License: https://creativecommons.org/licenses/by-nc/2.0/
Creator: https://www.flickr.com/people/30478819@N08/ License: https://creativecommons.org/licenses/by/2.0/
The reproduced paintings and photos are all in the public domain, but some of the source images of buttons are licensed under the share-alike license CC BY-NC-SA 2.0. The reproduced images above are therefore licensed under:
https://creativecommons.org/licenses/by-nc-sa/4.0/
I've bumped the license to the latest 4.0 version since these are compatible and because this is suggested by CC.