GIMP help needed

I got myself an iMac in the summer of 2012. I used to use windows so things are a little different. I downloaded GIMP and just now downloaded the updated version to see if it would fix my issue but I am still having issues.

on a windows PC, these files open in GIMP with no issues in JPG, GIF and PNG versions. If I go into the iMac GIMP, it tells me that the file is either corrupted or just isn't a JGP, GIF or PNG file when it really is. How can I fix this? I really would like to open these files in the iMac to work on them. I am trying to update some website things and I can't because I can't open these images on the iMac.

I don't want to seem like I'm being pushy here.... but my humble advice is to stop using GIMP and instead download the very nice "Pixelmator" from the Mac App store. It's one of the best bargains there is for $14.99 and very easy to use unlike the awkward and difficult to get working right "GIMP".

These pics you're trying, they do have extensions right? Like .png or .jpg