Is the photo black better or worse? Is there a way to do a post coat without having the residue of the transfer film show, or make the crackle effect?

Apply a quick sealing coat of the SuperSauce Solution.  Use single strokes quickly and do not brush hard or you will dissolve the image. Just flow it on and let it dry.  I have applied images to table tops and sealed them with 5 coats of the SuperSauce Solution. After that is down a Spar varnish can be applied to get a nice finish.