iPhone 4 water damage - how to recover data? My sister dropped her iPhone into water and made the fatal mistake of turning it on just a few hours later, thinking it was already dry. I put it into a bag with rice for days. She had taken a lot pictures within the last few weeks but neither connected it to her PC nor is she using iCloud/Photostream, so we wanted to save the pictures somehow.

The problem is, the phone only turned on when connected to a power socket - and only in the inital setup mode ("connect to itunes"). And even then it would restart after about half a minute.

Assuming the battery got fried, I ordered a new one from iFixit (the phone's out of warranty since a long time) and replaced it; of course I also cleaned the logic board in the process. Though I couldn't see any corrosion despite the water indicators being pink. The only difference now is that when connected to a PC, the Apple logo appears, but the phone reboots infinitely after several seconds. 

iTunes recognized it when entering DFU mode, but shows no additional data besides "iPhone" (capacity, iOS version, serial number).

Getting the phone to work again is secondary, we just want to save the pictures. Is there any way to get my data including all the pictures from iPhone?
Yes - programs like PhoneView can often read files from a device that iTunes insists must be restored. The iTunes function is to sync and clearly the OS is now in a state where it can't run itself or be updated, so the "correct" thing for iTunes is to ask for a restore.

I wouldn't say that you are guaranteed to get the photos back, but it's good you have the computer where the device normally syncs as that will have the encryption keys to unlock the storage if the device has data protection enabled.

You could also seek a quote from the usual data recovery services like:
- http://www.drivesaversdatarecovery.com
- http://www.krollontrack.com

You should expect to spend hundreds of dollars if you send the device in for recovery and you may have to pay if they recover anything - even one partial file - so be sure you understand what is being quoted before agreeing to a recovery attempt.