Exporting photos from Mac to PC

Hi

Recently got a MacBook Air, to replace an old Windows PC. Eventually got everything transferred over and decided that I could use my old PC as a backup for my pictures. Today I have exported all my pictures from 'Photos' to an external drive (FAT32) and can see the files on my Windows PC but when I go to look at the pictures they don't open - errors such as 'unknown or invalid JPEG marker' / 'not a file that Quicktime understands' / 'viewer doesn't support this file format'.

What have I done wrong? Is there a specific way I should be exporting these pictures so that they are viewable on a PC? Sort of restricts ever going back to a PC if Im not able to do so?

Thanks in advance

Nigel