If you want to change all of them uniformly, you can select them all, press G to group them, and then hold down the Ctrl key while you move the end of the last picture left to shorten them all or right to lengthen them.

Chances are you'll probably want to turn off auto-ripple while you do this.

Normally i never try to get the length right while importing the images. I'll make a guess somewhere in the ballpark, and then use the above method to get the length exactly right to match the music or whatever else is going on afterward.