How to send MY graphics to digital camera for slideshows?

I want to make custom graphics in Photoshop and put them on my still camera to run them to TV using the built-in slideshow function.

I save JPEGs of the proper pixel size, and re-use filenames (numbers) of images that came FROM the camera. (In fact, I pasted my image over a dummy photo that's still on the camera and re-saved directly there, with Photoshop's "Standard" JPEG option.)

But the camera refuses to recognize images I make--only its own. It shows a question mark on-screen instead.

What kinds of issues should I be researching to make this work? (It's a Pentax, FWIW.)

Thanks for any leads!

Next thing I'll try is different JPEG settings in Pshop, and including a Preview. Maybe different color profiles too.

It's low-priority though since my iPod Photo does the task automatically--with music