The photos were imported for 5 seconds each because apparently that's the duration currently set in the Photo Settings window, the window that opens when you press the Show Photo Settings button.
The shortest duration offered by the Photo Settings window is 00:03 — three frames. But there is a back door to import with a 1-frame duration. The trick is to use the iMovie feature that imports photos using the import duration — and all the other photo import settings — of the last imported clip you clicked on.
1. Click on any photo in the Photos list. Press the Show Photo Settings button. Turn OFF the Ken Burns checkbox in the Photo Settings window; drag the duration slider as far left as it will go; then press the Apply button. The new clip will have a duration of 3 frames. (Don't try to type the duration in the Photo Settings text box. The text box is buggy and you won't get the duration you want.)
2. Double-click on that new clip. In the Clip Info window that opens, change the duration to 0:00:01, which is one frame. Press the Set button. Now the clip will play for one frame.
3. In the Photos list, click on any photo. The default duration will be 00:03 in the Photo Settings window, which is the duration of your last import.
4. Now click on the new 1-frame clip, then click on any photo in the Photos list. Now the default duration will be 1 frame, not 3.
5. The photos you subsequently import will be 1 frame long until you change the duration, or click on a clip that's longer.
There's one other hurdle, importing the photos in the order you want them. Ideally you have them now in the same folder, and they are named in numerical order. To preserve the order, the best method is usually to use iMovie's File > Import command to import all at the same time. (Click on the first photo in the folder, then Shift-click on the last one.)
That method usually delivers them in the proper order. Dragging them from a Finder window — or from the photos list — usually will not.
Oh, and don't forget to set your iMovie preferences to import them to the Timeline, not the Clip pane.
If you can't get this to work then Yes, use QuickTime Pro's Import Sequence command. It lets you import all the photos with whatever duration you want. Save that movie, then import it to iMovie. Doing the type of movies you are, it's probably worth purchasing QT Pro for that one feature.
Karl