The quality of the PCB ARTwork depends highly on the sharpness and resolution of the original image. The bigger the size of the panel you want to order, the higher the resolution the original should have.
All pictures taken by a digital camera with a resolution of 5Mb or higher are acceptable. You can take the size of your file as a reference for the acceptable resolution:

goldPIXture format	Original file size
A5	0.2MB
A4	0.4MB
A3	0.6MB
A2	1.5MB

Old photographs that you'd like to turn into PCB ARTwork need to be scanned to create a digital image. Scanning them at 600 DPI yields good results for a 1:1 image (e.g., an original A4-size picture works well for A4-size PCB ARTwork) and acceptable results when scaling to a maximum factor of 4 times the original (e.g., an original A5-size picture yields acceptable results up to an A3-size PCB ARTwork).