Wrong extension or Image Format for output files

[pansk]

In this PR I changed the extension. Another fix could be adding a second parameter with System.Drawing.Imaging.ImageFormat.Bmp

[pansk]

It's funny because the Microsoft documentation itself is a bit confused on the subject, and in their code example they use .bmp as extension.
https://msdn.microsoft.com/en-us/library/ktx83wah(v=vs.110).aspx

[mef51]

had to make the same change .bmp --> .png

edit: works fine on windows, i only need to change the extension on ubuntu

[pansk]

I had the problem on Windows 10 and vs2015. Preview didn't complain, but another viewer did.

[mxgmn]

@pansk @mef51 Thanks. I moved everything to png. Didn't know that standard library works with pngs.