Hello!
I don't know if this has been reported already, but I've noticed that when a picture has its extension in uppercase, then when sharing the link (through the "share this photo with a friend" link), the thumbnail would not show in the email.
When the filename is Image.JPG, the url of the thumbnail in the mail is Image.jpg thus the problem.
I know this is a very small glitch in this particular case (hum... didn't do that one on purpose), but I had similar problems in the past (can't remember exactly in what particular situation, but I remember starting to rename all my files to put their extension in lowercase, which is pretty annoying... Especially when I forget to do it, which made me notice this small bug)
Wouldn't it be better to just create thumbnails that have the exact same extension case as the original filename, so it would prevent this kind of dead links?
But again, this is really a minor glitch in this awesome application si I hope you won't mind for me reporting it It's just that my camera puts all the extensions in uppercase, so that would be even better if I didn't have to think about renaming the files before uploading them.
Cheers,
Edit: and by the way, I remember that when a folder name has spaces in it, the link in the email doesn't work at all... but I know it's not good to have spaces in filenames or foldernames anyway, so I changed them. But just to let you know...