GIMP is an open source Image editing tool. There is a feature to create a slide show in GIMP with the output as Animated GIF format. It can be done in a few mouse clicks. My system is Ubuntu 10.04 with GIMP 2.6.8.
Here is the step by step tutorial on this.
Step-1: GIMP -> File -> Open as Layers… -> Multiple select your pictures
Step-2 (optional): If you want to resize all pictures at once, to create a smaller slide show animation: Image -> Scale Image… -> Adjust width and height as you like -> Scale.
Step-3: File -> Save As… -> Select File Type -> GIF Image (.gif) -> Save
Step-4: Export File dialog -> Save as Animation -> Export
Step-5: Save as GIF dialog -> Select “Use delay entered above for all frames” and “Use disposal entered above for all frames” -> Delay between frames where unspecified as time in millisecond as you like (I have selected 3seconds, so 3000ms) -> Save
That is it. Now you have your animated GIF file as slide show. The below shown California Nature Slide show created this way.