How to show width and height of image files in Finder's column view?
Icon View and Column View

Pressing Command+J or Right Clicking, while in the folder where the pictures are, will show the Options menú. Then Show View Options, and enable Show Item Info. This will show the image's dimensions below the file.

You could press Use as Default, if, well, you want it to be default.

List View

While in List view , by Right Clicking the list's title bar, you'll be able to choose extra information columns to show. As Koiyu explains here:


  The list of extra columns is relative to the folder's location. In order to get the dimensions available, the folder in question has to reside somewhere under ~/Pictures/. (Under ~/Music/ you could choose e.g. artist or album and under ~/Movies/ you could choose length or codecs and so on…)