2 Answers

In order to transfer pictures from a computer to a Samsung Galaxy S3, or vice versa, one needs to download and install a software for such transfer if using Windows (Makintosh users don't need that siftware). After the software is installed, connection between the two devices is made with a with a usb cable that can be seen in the following picture:
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After the connection is made, copying files from the computer to the Samsung Galaxy S3 can be done by copying the relevant files (highliting then with the computer mouse, and clicking 'Ctrl + c' in order to copy them), opening the Samsung Galaxy S3 link found on 'My compuer' folder that's on the computer, and pasting the files (by clicking 'Ctrl + v') at the desired location.
Downloading the software can be done at links listed at
https://www.google.co.il/search?q=samsung+galaxy+S3+usb+driver&hl=en&prmd=imvnsu&source=lnms&sa=X&ei=4PSIUL3nBoek0QWitoHQAw&ved=0CAkQ_AUoAA&biw=1366&bih=667. 

you can follow the instructions in user manual or online sites.