First of all, what is it you are trying to do? Display an image? Or process a user upload?
If you are trying to display the image, you cannot serve it out as a String as you do here:
Also, the image has to already exist if you want to display it.
If you are trying to let a user upload an image, you cannot process it in the doGet method, since those parameter values are passed in the URL. You have to do it in the doPost method. That's just the beginning, but there are other confusing, unclear things in your post. For example, why is the image data encoded as base 64? Where is this data coming from? There are plenty of tutorial out there on how to build an html form with a file upload field and how to extract the data in the servlet's doPost method, but I'm not sure these apply to your specific circumstances.
Likely the snippet of code you may be looking for. Not sure about base64 as I hardly ever have delt with it...only in practice. However, in that example it is getting the output stream from the response; presumably uploading it would be the next step ( from a Servlet anyway). Instead of outputing a string to the console you write it to a buffered image or engineer some other way to create the first argument of the static method write from class ImageIO. The output stream is from a Servlet so in this example it is getting uploaded to a client browser.