Lost Photos - Please Help

I know nothing about Macs so can't speak to the OS storage system.  How did you "download" the jpg files from your camera?  Did you do it through the Adobe Photo Downloader?  If so, the files should have been saved to your local hard drive from the memory card and they should still be there unless you have deleted them. If you select one of the missing images in the grid look and look in the Organizer's Information panel on the right side of the screen, there should be a Location link that tells you where the Organizer thinks the file is located.  Navigate to that location to see whether the file is there.

If you used the Organizer's Import from files and folders, and selected a memory card to import from, the files should be saved in a default Photos from Removable Media folder.  I don't know where that is on a Mac.  Perhaps you can use a file search tool to find the missing file.  Or, if you look in the Edit>Preferences>Files tab, the default location for Saved files should be identified and the Photos from Removable Media will be a sub-folder of the Adobe folder.

In any event, the Organizer only shows thumbnails of the photo file.  If the file has been deleted outside of the Organizer, the thumbnail remains.  That is what you are seeing.

If all else fails, it is generally possible to revive a photo from a reformatted memory card using some restore software.  The ones that work tend to be pricey, but if the images are important to you, they can be worth the money.