Trimming images to 16:10 for picture frame

Bought a picture frame a while ago, but it has a 16:10 aspect ratio and works best with images cropped to fit the same. Now, I am pretty sure can create a new file with the right ratio and guesstimate how big to make it based on what the desired area occupies on the original pic, then paste the original on top and move it around. However, is there a program which would let me lock aspect ratio of a selection to 16:10 but let me change its overall size at will?