Are these available somewhere as png/gif/jpg? They look really nice, and I would like to integrate them into my copy of Scid.

those are proprietary Chess.com pieces designed by me :)

Bummer. They are really nice!

"Proprietary"?  Copyright - for the specific images, okay - but dubious to defend against knockoffs.

You can get a graphic designer to to custom design a set easily converted to any file format, size, color..... that you want.  Unless you want something really custom in design the cost should be minimal for such a project.  I'm a programmer and don't do a lot of graphic work, but this is something that can sourced out with a quick turnaround.  It is very simple graphic work.

Integrating is separate issue, but anything is doable.