For those of you who didn't know, Dirty Bomb now supports Custom Crosshairs.
Since it's not exactly obvious how, here are the steps:
1. Navigate to the following directory: C:\Users\USERNAME
2. Place your crosshair file inside. Ensure it is 32x32 resolution and is named CustomCrosshair.png
Here is a ZIP archive of a bunch of crosshairs. [DOWNLOAD]
Keep in mind that you will have to either change the resolution of these crosshairs or transfer them to a 32x32 pixel image as these are all 100x100 pixels.