Service that syncs pictures only when charging I've been using Dropbox camera upload to automatically sync my pictures to my PC using my data plan (this part is important, I can't use a wifi).

However, this autosync eats up the battery like crazy, so I'd like it to only
sync when the phone is charging. I always connect my phone to its stand as soon
as I'm home, so I'll still have the pictures synced to my PC merely a few
minutes after getting home (without having to do anything manually).

Dropbox, unfortunately, doesn't offer this option (syncing only when charging).
Is there another photo-sync service/app that does? (picasa, g+, facebook, etc)
Google+ Instant Upload offers a setting to allow uploads only when the charger is connected.

Facebook's instant photo upload does not offer such an option.

The Gallery is linked to your Google Account and thus to Picasaweb. However, there's no option within the Gallery app to restrict data transfer only when the charger is connected.

Carbonite recently started offering a mobile app, and that includes backing up photos and videos. It offers an option to use battery or not for when uploading.

So Google+ or Carbonite Mobile are your choices. (At least from the photo auto-upload services I'm aware of.)