I have an application, currently written in C#, which can take a Base64-encoded string and turn it into an Image (a TIFF image in this case), and vice versa. In C# this is a
This should be a two-step process. Firstly, decode the base64 into pure binary (the bits you would have had if you loaded the TIFF from file). The first Google result for this looks to be pretty good.
Secondly, you'll need to convert those bits to a Bitmap object. I followed this example when I had to load images from a resource table.