Is it possible to take an image and have the GUI box fit to the size of the image?

I think you'll need to script it : doc

You can create it with a Rect that has the dimensions of the texture :

 var texture:Texture2D;
 var x:float;
 var y:float;
 var coeff:float = 1.0f;
 
 function OnGUI() {
     GUI.Box( Rect(x, y, texture.width*coeff, texture.height*coeff, texture));
 }

You can use the var "coeff" to adapt the size of your box without stretching the texture in one specific dimension.

NB : I didn't test the code :)