For now, consider some of these suggestions:
EXIF and XMP
Remove all EXIF and XMP data from your photos before you make them available publicly.
EXIF data is data stored in you image file pertaining to the camera, its settings for this photo and so on.
XMP data details all the processing steps you took after the photo was taken.
You don't want this info to go out with your photo.
Here is another well-written and easy-to-read article about EXIF and XMP:
If the issue arises of proving that a photo is yours, you want the EXIF and XMP data to be yours exclusively, you don't want the other party to have it.
Or modify you EXIF data and keep a record of the modifications you have made, so essentially you will be posting a photo with bogus EXIF data, but only you know what those modifications are:
This software will allow you to display and modify a variety of data.
DO NOT do this if the reason you are including the exif data when you post an image is to help others understand what you did. NEVER EVER mislead your viewers.
Keep a record of your image processing
If you use processing software that uses multiple layers, keep the file copy that retains the layers. For example, in Photoshop, keep the .psd files which have a record of all your layers and processing steps. But make sure you don't include the XMP data which records all this info. Even if you are using Picasa, always retain the Picasa.ini files that record your processing steps and use Picasa export to produce a lower resolution jpg for public distribution
Keep the highest resolution versions for yourself only
Keep your raw files, post lower resolution jpg or png or whatever format you want to use.
Here is an example of just a portion of the data recorded with an image in a jpg file, and most of this can be edited:
How about visible watermarks or copyright symbols
The short answer is they are effective only if they can't be removed without essentially ruining the photo. The smaller and less obvious they are, the easier it is to remove them. So yes, if they are so obvious the photo would be useless if they were removed, they would work. The downside is obviously that they would be of very little interest from a display/presentation perspective.
How about QR codes?
The problem with QR codes is they are visible. A QR code less than 1 cm by 1 cm becomes hard to read. So they are visible, and if they are visible, someone can clone over them or crop them out. As in other types of watermarks, they are most effective if removing them destroys the image.
There are many free QR code generators if you would like to try out QR codes, here is just one example:
What about digital watermarks such as Digimarc?
They are effective as long as the person copying a photo does not alter the image substantially. You can read all about Digimarc here:
and here is a broad overview:
And for some excellent white papers on watermarking and fingerprinting:
Keep all relevant information
If your photos include people, make sure you have all the appropriate release forms and that you have some system to relate the forms to the photos.
If you shoot scenes, keep notes on where each photo is taken. If your camera has GPS location capabilities, make sure it is turned on. Make sure your versions have the date and time. It is key that you retain information about the photo that is true, verifiable and that the other party does not have.
Please comment with additional ways in which a photographer can prove the photo is his/hers.