Is this the link to your blog? Can you give us a link to one of the posts you're talking about? It helps so we can get an idea of what we're looking at.
If that is your blog, I'm not seeing any major problems with images; they all appear on the posts so I think the files are okay, but some of the images are linked to the post url rather than the image url.
The image in the Fragments post, for instance, is linked to a .jpg stored in majhelenalander.files.wordpress.com; but the images in the posts below it all link to the posts, not the image files, which is why when you click on them, you can't see it enlarged. You haven't made any changes to your image files stored on WP, re-uploaded them or something like that?
I'd suggest editing a couple of the posts and see if you can change the link to the image file; I don't know what would have changed it if it was working before, but as the images still appear with your posts, I'm hoping that will fix it. Or you could try uploading the images to a host like Flickr and inserting them into your posts with the picture icon (#12); it'll save your upload space and the url should never change.