It appears that this is a feature of the Google Photos app, and is used as an indication that the photo is not backed up.
If you have a G+ account and are backing up your phones photos to it, pictures will appear dimmed (after a second or so) and then brighten again once it's been uploaded.
I don't understand why this feature has been implemented in such a way, after all, I'm mostly out when taking snaps on my phone and don't want to use my data connection to upload. Meaning that I either have to put up with the stupid dimming, or use another application (I prefer the features of the Photos application) until I can find a wifi connection to do the transfers.
I'd much rather have an icon indicator on the screen that shows me whether a picture has been backed up.
Hopefully there will be enough complaints to have this rectified in a later version of Android OS.