If you are creating the slideshow solely in the Premiere Elements 9.0/9.0.1 Editor....
After import of the source files to the Timeline....
Select all the clips at one time, right click anywhere in the selection, select Time Stretch, set the duration 00;00;00;00 (hours;minutes;seconds;frames) and OK out of there.
To select the clips at one time
a. As you hold down the Shift key, click on the clips wanted in the selection
b. With the mouse cursor draw a rectangle around the clips to select them
For before import...
See Edit Menu/Preferences/General and the setting for Still Image Default Duration...that setting applies only to the photos that you import after the setting has been made.
Any questions or need clarification, please ask.