Negative Images In PDF Files

I created a pdf file for each entry of a blog I used to have, right before I pulled it off line.  Not a problem.

I then took all the individual entries and combined them into one file.  When I did this, however, many of the pictures showed up as negatives.  Not all of them. Can't figure out why. There is no problem with the individual files; all the pics appear as normal in those.

Any help is most appreciated.