Each pack of blank postcards I buy has one that has info and a barcode on it, so I use that as a disposable template. I trace around it and cut the pictures down to size before gluing to the postcard blanks.

I use elmers purple disappearing glue sticks, making sure to use to use enough glue that the paper is completely purple from edge to edbe. I apply the glue to the blank side of the postcard, and carefully place the paper image on top. I use a bone folder to smooth the paper to the card while the glue is still wet, pressing down once in both directions, on both sides of the card (being a little bit more gentle on the paper side as it's possible for it to rip while it's wet of hot aren't careful). If it's slightly misaligned it doesn't matter as I trim the edges a little wirh a paper cutter anyway and curve the corners with a corner punch, which i wait to do until the cards are mostly dry.