When I'm importing fixed pictures upright into iMovie these will be cutter and they are not visible completely

If you import them from iPhoto they should retain the proper size.

In iMovie, on the middle toolbar you should see the Rotate, Crop, Ken Burns Tool.

Select your photo in the Project and use this tool to select FIT to keep the entire picture and fill the extra space with black bars (letterboxing). Use CROP to make the picture fill the frame by chopping off the top ad bottom. Choose Ken Burns to create a motion effect.