Image dimensions infos on Lion Finder Columns view
It looks like this has to do with Spotlight. Finder's knowledge of additional file information is integrated with Spotlight, disabling it, or adding locations to the privacy  list will prevent Finder from reporting file information. This includes image dimensions.



If Finder is not reporting "More Info," make sure that directory is not blacklisted by Spotlight. In the screenshot above, anything found in the "Private" folder will show a "--" under the More Info section. This is by design.