You could also do this at the same time as moving both your contacts and your music.
First you need to ensure that you have made a back up of your most recent photos and videos on your iPhone.
1. Download the Easy Phone Sync app to your PC or Mac. Here's a useful video that talks you through this.
2. Make sure you have the Gallery app in your Favourite Apps Menu on Galaxy S3 and then also download the Easy Phone Sync app - either from the phone itself or via Google Play on your computer.
Go to your PC and follow the instructions as you go through the menus there and then watch as the iPhone images start to appear on the Samsung in the Gallery.
Renaming your photos and videos for more clarity
It was a right old mess in my Gallery as all the files had very generic names like IMG_0113 so it was great to be able to go in and Rename things. You just select the item and then click the icon to the right of the central Home button. This will bring up a menu which offers the Rename option and Bob's your Uncle, you can identify your various memories - photos or videos - with ease.
Make sure you save!