Styles are usually created from layer effects rather than actual files. However, as a workaround, open your JPG, go to Edit > Define Pattern... Give it a name, and click ok.
Create a new layer, then go to Layer > Layer Style > Pattern Overlay... Choose your pattern and click ok, although you mightn't notice anything just yet.
Now find an empty space in your Styles palette (the cursor will change to a paintbucket) and click. Try it a layer in another image and you'll see your pattern tiled across non-transparent areas.