This is a two part hexagonal picture frame.

It's outer size is 20cm. So you can cut A4 printed photos  to fit in it.

Otherwise you can use the slicer scaling function for other dimensions.

Or you can modify the openSCAD file accordingly.

The back part can be sticked to the wall by means of adhesive strips. You can change the picture without renewing the glue strip. But if you want to, you can reach the strip through the big holes.

I printed mine in matt PLA.

Have fun!