How do you find out what an old picture is worth?

The most famous solution is to take it to the antique roadshow, but depending on the type, condition and contents of the picture, you may be able to find some groups that buy and trade antique/vintage photos on the internet who would make your offers or tell you how much they think it's worth.

Try sites like

ioffer.com

etsy.com

collectorsweekly.com

flickr.com

antiquepooch.com

You may also take your photo to a local vintage shop to have them appraise it.