This a dirty hack to display Day One's photos on OS X in an HTML page, in a similar view as the iOS version of the app.
It's a very simple shell script (+ a few css style) that looks into iCloud's files to find DayOne pictures.
If you use Dropbox instead, change PATH2ENTRY= value to reflect the path to your DayOne photos folder. Something like ~/Dropbox/Apps/Day\ One/Journal.dayone/photos.
If you don't like command line, the Automator Services should work fine (unzip it and save it under ~/Libray/Services).
Tested under Mountain Lion 10.8.1.
More info (in French), here : Galerie photos Day One sous OS X.