Finding no captions for a couple of images in a club competition gallery, I identified this as the reason.
To get the winners' pictures presented first, I rename them before using Gallery Mage so that if (let us say) Smith.jpg is the winner and Jones.jpg is second they become 1Smith.jpg and 2Jones.jpg, and are sorted into first and second positions in the gallery, with the remainder of the images following in alphabetical order.
This month, two images tied, and I called them 1=Smith.jpg and 1=Jones.jpg. In processing these images, Gallery Mage converted the "=" sign to "_" in the filenames, but kept the "=" sign in picdesc.txt.
Obviously, it's easy to edit picdesc.txt once the problem has been identified, but it would be helpful to have this inconsistency fixed.