It could different things to different people. But for those who'll see it for the first time will initially think of the cowboy's faith in God. The image symbolizes the Christianity in the cowboy community.
via kwout
The stickers are often printed with "We ride on faith" or "In God we trust" taglines.
The image is also being linked to the "Cowboy Church".
A contributor of AirCoolers.org by the name Turtle explained the decal's connection to the church:
The Cowboy at the Cross sticker original spread out of the "Cowboy Church" & the stickers surfaced around a decade ago as Christian pop culture peddlers of the day were plagiarizing nearly every secular icon out there. The Cowboys happened to modify one that suited them from a sticker of a boy kneeling at a cross which yes was a plagiarization the whole Calvin & Hobbes sticker run. Aside from that Cowboy Church originated within the rodeo circuits as a bible studies with the circuit riders that were always at events on weekends during status quo church services. These eventually morphed into doing church outside of the box with the cowboys making church fit their needs. Most of them felt out of place in contemporary churches so they established their own organization & started holding services in local community rodeo arenas in the late 80's early 90's where they could rope & ride after service was over.
For a directory of cowboy churches and cowboy ministries compiled for your convenience, go to Cowboychurch.net.