adding a caption to a photo

There are two possible meanings of what you're requesting. Do you want to modify the image file to include a caption, or do you want a caption to be displayed whenever you view the image, without modifying the image?

To be displayed whenever the image is viewed

Laurie:

This is a limitation with the concept of files. Most image file formats do not support a "caption" tag that is understood by the viewer. This is because this is beyond the requirements for the file. The file must simply provide a 2D grid of Red-Green-Blue data. It is not meant to be a document system itself

What you *can* do is "burn in" the caption to the image. Simply use a tool like GIMP or any other image editor to modify the image data itself, such that your caption *is* the image.

If you want to understand a little more, and why this is not a common functionality of any image format, read about "raster data" in wiki:

https://secure.wikimedia.org/wikipedia/en/wiki/Raster_graphics

Some image viewing programs support comments for images that appear whenever the images are viewed; these can be used as captions (though they are not transferred with the image, when you send the image to someone else). Is that more along the lines of what you're looking for? Eye of Gnome does not support this, but you can find other viewers that do.