How do I change the grid background in Doombuilder 2?

I want to change the black background to something else. How do I do this? When I try to pick the picture, it won't show up. I don't think it will allow JPEGs. What file format do I need to use?

It tells you right there which file formats are supported

Mine doesn't say that. It just says "all supported images". Thank you though.

TraceOfSpades said:

Mine doesn't say that.

Mine neither. Why Kappes Buur's one does?

Strange that.
I checked in r1604, r1630 and r1715
http://devbuilds.drdteam.org/doombuilder2/
and they all show the supported formats line.

Strange.

Well, either way...let the record show that .PNG works fine.

Maybe it's your operating system or file browser rather than Doom Builder that doesn't read the suffixes? Is it possible? (I have Windows 7 Home Premium.)

I have windows 8.1, unfortunately.