Force OS X Finder to always show dimensions of images in sidebar
You might need to re-index the directories on your system by doing the following:


Go to Apple &gt; System Preferences &gt; Spotlight and chose the Privacy tab.
Drag a directory (or an entire volume) into the list, and confirm the action
Remove the directory (or volume) from the list via - it, which will trigger a re-index of the directory or volume.


Source