Under Options / Preferences / Editing, you can set new still image length. This affects how long the pictures will be when you add them to the timeline. This could be very useful for you for future work.

However, if they're all already on the timeline, you can select all of them, press G to group them, then hold the Ctrl key down while you slide the end of the last image to the left. This will shorten all of them all at once.