I've toyed around trying to make useful sprays showing a bit of text or whatnot. However, they become low-res once I spray them ingame.
For example, this spray:
...becomes this ingame:
How can I fix this?
Based upon the rest of how the graphics look (looking at the texture of the floor and of the wrench), it may be your graphic settings. It's just the way the game handles settings all around. Think of it this way, it applies a generic "template" of how to handle all graphics at certain settings. If you can up your video settings it should look a lot better.
Your spray imported as a BMP looks fine on higher settings: