What you want to do is change the still image duration.
If you already have them imported and on the Timeline, then
1. Select (all at one time) all the 6 seconds clips that you want to change to 1 second clips.
2. Right click anywhere in the selection, select Time Stretch, and, in the Time Stretch dialog, change the duration field setting from
00;00;06;00 (or whatever it is) to 00;00;01;00 (readings is hours;minutes;seconds;frames).
Before you import the photos, you can
1. Change the Still Image Default Duration in preferences from 150 frames (5 seconds in 30 frames per second system)
to 30 frames (1 second in 30 frames per second system). See Edit Menu/Preferences/General.
2. Then import the photos. They will all arrive on the Timeline with the 1 second duration.
3. That duration in preferences will keep that settings until you change it.
Please review and consider.
Thanks for letting us know the results. Glad that worked for you.