craftgrrl | no-framed photographs.

im prefacing my question with "i promise i've searched the memories".
with that said, im guessing this quesion has been answered before, but unfortunately im not sure of all the correct terminology to search for, so i'm having little luck.

im curious as to what type of glue i can use on professional grade photo paper to secure a picture to a piece of wood without eventually causing damage to the photo or its quality.

a friend and i had some photos taken of our tattoos for our tattoo artist [late christmas] and we want a very "diy"-esque frame/mount for the photo.

i wanted to hopefully get a piece of 1/4" thick walnut or cherry wood to lay the photo on top of.
the wood will also be cut to the same measurements of the photo... so there will be no type of frame or border.

ive also heard that there is some sort of liquid that can be used to coat the top of the photo to prevent scratches over time.
im incredibly hesitant to paint anything over the top of the photograph,... but perhaps its some sort of spray?

if anyone could let me know what glue wouldnt harm the photo, but hold up well, and also if there is any kind of protectant that can be used directly on top of the photo [in lieu of glass] .. i would be so grateful!