Arcticsid asked the Photo Editing forum for a way to add captions to his photos. "In the old days we could write a description on the back."
I'm assuming you're saving and sharing those photos as .jpg files. And that gives you an advantage, because jpegs have captions build in as part of the format's metadata.
Metadata is a particular kind of data that explains a file's content, and most file types contain their own metadata fields. For instance, a jpeg's metadata includes the camera model, resolution, the date the photo was taken, and other bits of information. To view and potentially change a file's metadata, right-click it, select Properties, and click the Details tab.
If you want to add a caption to a photo, use the jpeg format's Title field--the very first one on that Details tab. Point the mouse cursor to the blank space to the right of the word Title. A field will appear; click there and type your description.
If you want to use the same caption for multiple photos, just do that en masse. Select all of the files in question, then do what I described above.
Both Picasa or Photo Gallery display the contents of the Title field as the photo's caption. And if you give a photo a caption in either of these programs, your words will be saved in the file's Title field. (Photo Gallery will save those words in both the Title and Subject fields.)
If you upload your photos to Flickr, your Title field travels with it, and becomes a caption there, to.
Read the original forum discussion.