Could be paper, could be a Polaroid frame – up to you!
Source Chaos
Background pattern originally a PNG drawn in Paint.net
Source Firkin
Prismatic Polyskelion Pattern No Background
Source GDJ
A seamless chequerboard pattern formed from a tile that can be had in Inkscape by selecting the rectangle and using shift+alt+i. Alternative colour scheme.
Source Firkin
Prismatic Curved Diamond Pattern 4 No Background
Source GDJ
Produced using the clouds, flames and glass blocks plug-ins in Paint.net and the resulting .PNG vectorised with Vector Magic.
Source Firkin
Prismatic Curved Diamond Pattern 8 No Background
Source GDJ
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
A seamless background pattern with a texture of wood planks. This wood background pattern has vertically arranged planks. You may try to rotate it 90°, to see how it will look like when the wood planks are arranged horizontally.
Source V. Hartikainen
To get the tile this is based on select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
To get the tile this is made up from select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Dark blue concrete wall with some small dust spots.
Source Atle Mo
A seamless background tile of aged paper with shabby look.
Source V. Hartikainen
A seamless pattern of dark bricks. Maybe it's not very realistic, but it looks good in my opinion.
Source V. Hartikainen
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
Prismatic Isometric Cube Wireframe Pattern No Background
Source GDJ
Abstract Tiled Background Extended 10
Source GDJ
A pale yellow background pattern with vertical stripes. The stripes are partially faded. I think this background image turned out pretty well, especially those faded stripes.
Source V. Hartikainen
A background pattern inspired by designs seen in 'Burghley. The Life of William Cecil', William Charlton, 1857.
Source Firkin
No idea what Nistri means, but it’s a crisp little pattern nonetheless.
Source Markus Reiter
Geometric triangles seem to be quite hot these days.
Source Pixeden