Commandline utility to convert any GDI+ supported image (BMP, GIF, JPEG, PNG, TIFF) to XNB image. The version of XNB file will be Version 4 (XNA Framework 3.1). Mainly created to modify Defy Gravity -textures, but should work for all games created with XNA Framework 3.1 (and might work with XNA Framework 4.0, as it should be backwards compatible).
The resulting XNB files are uncompressed, so for bigger images they could get quite big indeed.
Download the executable from the latest release and copy it to your chosen location.
In commandline with ImageToXNB (or in the same directory), run
ImageToXNB <source> [<destination>]
Destination is optional, and in case it is omitted, the filename is the same with the extension replaced with "xnb".
If I have time, I could investigate implementing XNB compression. Other than that, there is not much to this. A simple program, not much to improve.