I've not had problems with JPG files either, but read posts from some who
have....
If you need to swap them out, you can do it without having to re-edit, but
you would need to put the new set of pix in the same folder location and
with the same names, including the extension.... for example:
1 - you use c:\images\picture1.jpg in the movie.. and then want to change it
to a .bmp.
2 - convert the file to a .bmp but name it picture1.jpg
3 - replace the picture1.jpg file with the new one of the same name, but
really a .bmp file.
When Movie Maker opens a project, it only checks that a file is there with
that name, and when it renders the movie, it takes the info from the file
and puts it into the movie... it never checks to see if the actual contents
of the image matches what was there when you put it in the project.