Go the the Preferences, under the Edit menu on the PC version of the program.
Once you change this duration, note that it will only affect photos added to your project AFTER you make the change. Photos added to your project before you changed this preference will maintain their 5 second length.
Welcome to the forum.
Steve has pointed you to the Edit>Preferences>Still Duration setting, and I believe that is exactly what you are asking for.
If you have already Imported Stills, with the default 05 sec. Duration, and placed them on the Timeline, one can just Click+drag on the Head, or Tail of the Still (Clip) in the Timeline, to alter the Duration for those. I find that this editing process is easier, with the Info Panel (Window>Info) open, to see the exact Duration, of the change.
One could also apply the Effect>Speed/Duration to those, but I find Click+drag to be the most effective. However, if one calculates the Speed Duration Effect, to get what they want, that Effect w/ its adjustments, can then easily be added to all of the Clips (Stills), to alter them en masse.
Just wanted to add a bit more Premiere Elements detail to what has been suggested already....
Premiere Elements 11 Windows
1. Setting the Still Image Default Duration BEFORE you use Add Media/ to import your media into the project.
Edit Menu/Preferences/General and the option toward the top which reads
"Still Image Default Duration". The units there are in "frames". So, you will probably see 150 listed as the default.
For NTSC (30 frames per second frame rate), then 150 frames = 5 seconds, 300 frames = 10 seconds....
For PAL (25 frames per second frame rate), then 150 frames = 6 seconds, 300 frames = 12 seconds.
2. Once you have imported stills, you can change the AFTER duration using the Time Stretch Tool (found under the Tools Menu or Tools Tab/Expert workspace).
a. Select all the stills, right click anywhere in the highlight, select Time Stretch, and change the duration there from 00;00;05;00 (hours;minutes;seconds;frames) to 00;00;10;00 if you wanted a 10 second duration for all the stills.
Please let us know if that worked for you.