Make a homemade photo sensor?
One option would be 


Mount a small photo-sensor on an X-Y carriage positioned on the focal plane of your "camera". 
The shutter would be left open permanently and the sensor stepped across each line in turn taking as many samples as you want to construct your image. 
An analog grey-scale reading could be taken at each point, shifted into memory and a grey-scale bitmap created from the results.


Here's a variation on the idea:


Figure 1: A single-pixel camera on a pan and tilt mechanism. Source: Arduining.com.