The easiest way is to output your slideshow as a video and then import that video back into your project and use Time Stretch to slow or speed up the playback.
How you do that depends on how you plan to use this slideshow. Will it be output on a DVD or BluRay disc? Uploaded to YouTube or Vimeo?
In addition to Steve's method, you can change the duration default prior to starting a project. Edit > Preferences > General will display an option to change the "Still Image Default Duration".
It will take either a little math or some experimentation to find the setting you like. For example, with a project setting of 60 frames per second, 120 frame duration is two seconds. Set at 1, 60 images would display in in one second.
Steve's method has the advantage of being adjustable within the project. It has the theoretical disadvantage of a potential loss of image quality by adding an output rendering cycle.
Thanks Steve. That seems to be correct, since I have no problems with speeding up or down ordinary video clips and takes with Time Remapping.
And also thanks a lot to Bill - your answer brought me to succeed with the stop-video, so that I understod what to do. Now it works for me!