This quest does need repaired. it's one of the major reasons I haven't started the Valencia quest lines.
It's quite easy if you are aware of the issue. Just align your minimap to the pictures above and you will dig them up in no time!
I've heard from a lot of people that tried that, that the rocks and other stuff in those pictures doesn't always show up, and that the actual space you can find the coins is so small that it has taken some of them hours and over 500 shovels to find them.
If you follow this it is literally a lifesaver. http://forum.blackdesertonline.com/index.php?/topic/111239-golden-desert-coin/&page=3