VCarve will let you import a jpg and then you can use the trace function to create the lines you’ll later turn into toolpaths. You’ll likely need to edit the trace results using the node editor but it’s not terribly difficult - just can be tedious when the jpg isn’t high quality.
And yes there are premade design files out there for guitar bodies and necks. I think I’ve seen some free ones as well being discussed in some of the YT videos. I’d do a search on “YouTube guitar VCarve” and see what pops up. Vectric has a lot of files themselves as well as videos and a good forum as well so you should be able find what you need.