I’ve created background image using this technique: How can I display a flat background - 2D image, not a skybox - behind everything in my camera? - Unity Answers and it’s working fine. Obviously it’s possible to replace GUITexture with another one but this will be very sharp change. Question is whether it’s possible to change background image with effect such as fade?
If you want to smoothly fade to another image, create another image in front of your old one, and then smoothly fade its alpha value in its colour up from zero. Then when it reaches one, delete the old image. Done!