Re-Importing photos into iMovie?

Hello,

This is a difficult question to explain, but I'll give it my best shot: So I used to use windows (and windows movie maker) and I started to notice that if I drew a picture (say, on MSPaint) and then imported it into movie maker, it would be all okay. Then if I went back and made a change to the picture, then replaced the old file with the new one, it would also change the picture in my movie on movie maker. (as in, it would replace the old imported picture with the new-replaced-file picture.) I tried doing this in iMovie, but for some reason, whilst the file changed, the picture which was already in the movie did not. Is there some way I can my movie pictures to change if their files change, or do I have to re-import them each time? (I only ask because It's very useful for animtion, if you have all the frames the lengths you want them as sketches, then you can just easily replace them with their fully-coloured and shaded counterparts)

Any ideas? Thankyou! - Roxina100