How do I change the grid background in Doombuilder 2?

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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?

[Kappes Buur]

It tells you right there which file formats are supported

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

[scifista42]

TraceOfSpades said:

Mine doesn't say that.

Mine neither. Why Kappes Buur's one does?

[Kappes Buur]

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

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Strange.

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

[scifista42]

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.)

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I have windows 8.1, unfortunately.