Since you say bg1/bg2, I assume you are using my script? Bg1 and bg2 are temporary files created during the image overlay process, and are cleaned up at the end to not take up space.

To use only one image, the simple way is to replace space.png with your desired png, and set the temperatures to select summer/winter to something very high or low respectively. For example: Set summer to 300 and winter to -300. Then it will always choose the last default (space.png) which you can make any image you want.