Here’s a good link to show what kind of images work well with 3D.
http://bit.ly/2zaLQFX (short cut so you don’t have to read the thread if ya don’t want )
Here’s a bit more info about it
and some more examples here too
I remove masking before engraving as it just turns gummy and can be hard to get off. I just clean the wood residue off with alcohol or orange cleaner. To me, the remaining discoloration adds depth and character to the engravings. YMMV of course .