Replace stills with better quality images without losing Ken burns and timing settings

I took screen shots of an illustrator file to make into a quick promo video. I saved them to the desktop and draged them into iMovie. I realize now that that is what is most likely causing the images to look terrible in the video. They are blurry and the video is jerky.

Is there a way to EITHER:

1) improve the image quality within iMovie (change scale, etc)

or

2) replace the images with better quality ones without losing the ken burns, timing, and music settings I spent so much time on?

Simply draging the new images into iMovie and selecting "replace" doesn't work.

What is the best way to import still images to prevent this happening again?

I am using iMovie 11 v 9.0.8

THANKS!