some jpg's created under Windows have turned black on my Mac

Hi all,

I have a problem with jpg images that have turned black in my PowerPoint slides. Now there are some similar posts on the internet about jpg's turning black when they are embedded in other programs (Word, PowerPoint), but the details are always different. And in any case I have not found a solution to the problem yet. Maybe one of you knows how to solve the problem. I am providing as many details as possible:

-The PowerPoint slides where created with PowerPoint 2007 for Windows. The jpg's were created in CorelDraw 14 using the export option. Then they were imported into PowerPoint using the insert->picture option.
-I think (but I am not absolutely sure about this) that at some moment the slides worked fine in PowerPoint 2008 for Mac. Now, however, some of the jpg's have turned black.
-I don't think it's just a problem of PowerPoint because the jpg's themselves are black. They're also black when I open them with Preview and iPhoto.
-Under Windows (Parallels Desktop) the jpg's just look fine. Also, if I remake the jpg's (by exporting) with CorelDraw, everything is fine. However, I don't want to have to remake all these jpg's.

To me it's just a mystery why some of the jpg's are black while others are fine. Can anybody help me out on this?

Best,
Ariel

Have they has any sort of image adjustment done to them within PowerPoint?

The only adjustment I may have done to them is resizing. But anyway the original jpg is also black.

Another thing I found:
If I open the original powerpoint slide in PowerPoint for Windows the jpg looks fine. Then I can right-click on the image and "save as picture". Next in PowerPoint for Mac I right-click on the black jpg, choose "change picture" and then select the saved image. Everything is fine. However, I don't want to do this manually for tens of jpg's.