Here’s an easy way to re-size your image file in Photoshop, without sacrificing the quality of the image itself. TRUST ME, this won’t degrade the quality whatsoever.
Just 4 more (quick and easy) steps after the jump…
Step two. Go to the Image Menu and select “image size”.
Step three. The image size window opens up. On the top, change pixel dimensions so that it is showing the percentage, not the number of pixels.
Step four. Change percentage from 100% to 50%, and hit OK. 
Step five. Save your file as “file name-halfscale”. A jpeg, tif or psd works best.
(As for your dpi — 72 is fine. 180 is fine. 300 is fine. Anything higher, just isn’t necessary.)