Missing using System.Drawing.Imaging for Base64 encoding


I need some base64 > http://en.wikipedia.org/wiki/Base64 encoding of images.
Therefore I need to use do some use:

using System.Drawing;
using System.Drawing.Imaging;

But I am not allowed to do so, even though I have net 4.5 installed which is afaik required to use those libs.

Any Ideas?

OK simple solution,
I had to set the reference to System.Drawing; and set the Target Framework to Net 4.5.