The game's graphics files can be modified with a few tools. To extract graphics you can use a tool like XnaConvert, see: https://github.com/Athari/XnaConvert

However, converting images back to XNB files the game understands is a bit more problematic. As far as I know, there isn't readily in circulation any XNB Content Pipeline tool that supports XNA Framework 3.1 (XNB Version 4) anymore (which Defy Gravity uses), and as such, things get a bit hairy.

However, I went and reverse-engineered the format with the help of of some disassemblers, hex editors, spec sheets for XNA Framework 4.0 XNB files (XNB Version 5), a lot of time and dumb luck, and I have whipped up a quick commandline tool to fill that void.

You can grab my ImageToXNB from here: https://github.com/jkarkkainen/ImageToXNB

Please do post your graphics mods if you happen to do any, whether it's awesome or crappy or stupid or funny or anything at all!

Here is a sample reticle to replace the original with:

Replacement high contrast reticle

Just replace the file in reticle.xnb in DefyGravity\Content\Sprites\Gui