In fact, if you're really clever, you can create multiple pic files that when used in different combinations, can create a polynomially greater number of total pictures.
EX:
----------------------------
| **** ** ***** *(*|
| ** ** *** |
----------------------------
night pic (upper half)
----------------------------
| -*- |
| /|\ |
----------------------------
day pic (upper half)
----------------------------
|/\/\/\/\/\/\/\/\/\/\/\/\/\|
----------------------------
mountains (lower half)
This is a small and crappy example, but you could use these three pics to make the mountains be under a day-time or night-time sky. With additional lower-half pics, the same day or night sky would double the possibilities. Just a suggestion.