You'll have to know the RGB values from the image as NX doesn't use images for background colors, just a color palette. Once that is done, and depending upon how you have NX set up you will do one of 2 things or possibly both:
Edit your Customer Defaults (Menu -> File -> Utilities -> Customer Defaults; on the Customer Defaults dialog -> Gateway -> Visualization -> Background Color tab) such that the Top & Bottom RGB values match what you have in your sample image.
Edit your each template such that the Top & Bottom RGB values match what you have in your sample image (Menu -> Preferences -> Background). Set the Background to Graduated if it's not already, click the Top color/Bottom color and enter the RGB values accordingly.
If you have to take the Customer Defaults route, you will have to restart NX for the settings to take effect. This will NOT change existing (saved) files - you will have to do that manually using route #2 for each file.
You CAN bring in an image to use as a reference for "tracing", like an artistic sketch or drawing of a part so that you can build surfaces. You can also place images on drawings (like for company logos, bar codes, etc.).