The max DPI that the scanner can obtain from a digitally printed photo is the DPI at which the picture has been printed. Scanning with a DPI value higher than this will not result in an improvement of the picture quality.
In any case if you don't know the DPI resolution of your printed picture you can follow these simple rules to obtain a good scanning.
Consider the dimension of the picture that you need to print (after scanning), let's say 5x7. Usually 300dpi for a printed picture is considered good quality.
Multiply 7 times 300 = 2100. Divide by 6 (height of your orignal picture) and you get the scanning resolution 350dpi.
For the secondo picture:
If you consider to print it in the same format 5x7,
Multiply 7 times 300 = 2100. Divide by 7 (height of your orignal picture) and you get the scanning resolution 300dpi.
Hope this is clear enough