If you decide to do the duration change after you imported photos to the Timeline....
In Premiere Elements 11, 12, or 13 on Windows 7, 8, or 8.1 64 bit, right click the photo, select Time Stretch, and change the Time Stretch dialog's duration field.
If your duration is now 5 seconds, the reading in the Time Stretch dialog would be 00;00;05;00 (hours;minutes;seconds;frames). If you wanted to increase that duration to 7 seconds, the you would change the duration in Time Stretch dialog to 00;00;07;00.
If you wanted to do more than one at one time, the select all the clips involved, right click anywhere in the selection, select Time Stretch, and then change the duration in the Time Stretch dialog field.
If you decide to do the duration change before you import photos to the Timeline....
You can set the Still Image Default Duration in Edit Menu/Preferences/General so that all photos imported AFTER that setting will have the duration the you set.
The default duration is 5 second and given in units of frames, so 150 frames. If you wanted to change that setting to represents 10 seconds duration, then you would change the 150 frames to 300 frames.
Please let us know if you have any questions about the above.
Thank you so much