Graphics that are in PoG but not in the Coldstone graphics library are generally not OK to use in your own creations, any more than graphics you pulled out of any other game would be. It's been stated in the past that certain graphics (such as the Sea Giants and Guardian in PoG) were intentionally omitted from the Coldstone library in order to keep PoG somewhat unique.
For something as simple as the "road sign" graphic, it might have been left out of the Coldstone library inadvertently. You could try e-mailing Ambrosia about the possibility of being permitted to use it in your own work.
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'You can either be on the stage, just a performer, just going through the lines...or you can be outside it, and know how the script works, where the scenery hangs, and where the trapdoors are. Isn't that better?'
-- T. Pratchett