If you could share a link to your directory and that entry specifically that would be of great help. As of right now, I can only guess… Try uploading a smaller image file. When I say smaller, I do not mean file size. Make the actual height and width smaller. Multi mega pixel images can cause the server to run out of resources while trying to process it. It could work in the admin simply because less images are being shown which mean less processing and more resources are available to process the image for the admin.
Hope this helps, let me know.