I'm looking to work out why some photos I have taken with my phone have strange images of a girl in them, when she clearly shouldnt be there. Can a photo taken with a normal camera phone double expose and cause this? Can existing images on the sd card remain there and overlay onto new pics taken? many thanks.
An example would be helpful, but if you've had your phone awhile, there is a strong possibility that the flash storage in your phone has reached what's called its "write cycle limit" and has started malfunctioning. If your photos are being stored on a removable device, replacing it with a new one will likely solve the problem.
Any given chunk of storage (called a "block") on a flash memory device can be re-written a limited number of times before it is no longer able to store new data. The circuitry in these devices uses a technique called wear leveling to spread writes among the blocks as evenly as it can in an effort to allow the device to work reliably for as long as possible by preventing frequently-written blocks from reaching their limits.
Manufacturers have a rough idea about how many write cycles their memory can take, but it isn't an exact science. Blocks eventually fail, data gets written but it doesn't "take," whatever data was there previously remains, and whatever wrote it is none the wiser unless it takes the time to verify it and squawk about a failure.
What you may be seeing is ranges of blocks that fall within your new images reading back data from the old ones. This can happen in ways that are perfectly plausible to the software the decodes the images and displays them on the screen even though you may find it incongruous.
I would suggest that if you have data on the device you value, don't write anything else to it and copy it off now. As more blocks fail, the problem will continue to get worse, and a bad write will eventually happen someplace critical to the organization of the file system and the whole thing will quit working.