Comment 4 for bug 278941

I ran into the same issue and I worked it out. I figured I'd post what I found to see if it helps.

There's really two issues and one was right in the man page for glslideshow. From the man page:
---
       -pan seconds
               How long each pan-and-zoom should last. Default 6 seconds.

               With the default settings of -pan 6 -duration 30, each image
               will be displayed five times (30/6), and then a new image will
               be loaded. If you want a new image to be loaded at each fade,
               then set -pan and -duration to the same value.
---

This is the exact behavior I was seeing and sounds like what the other users are describing... So I modified '/usr/share/applications/screensavers/glslideshow.desktop' and set both duration and pan to the same value and it appear to be doing the same thing...

However, what I found was that when I tested by going to "Lock Screen" it wasn't picking up the changes even though the screen saver preferences panel did (System->Preferences->Screensaver). (Using something that modifies the visual like '-fps' is helpful to see if changes are being picked up or not.) What I had to do was restart gdm after I modified the glslideshow.desktop file. The xscreensaver-demo tool seemed to introduce uncertainty... Especially since the interface provides sliders that don't show the exact set value, and as the man page states above unless duration and pan are set to the same value the "same image cycling" issue appears... So, I nuked the .xscreensaver file in my home dir and added the 'imageDirectory:' parameter by hand. Restarted GDM and everything works now.

Not really a bug, the system just does something different then what (some) users expect.

I hope this helps.