We had a problem with them on our side again and it must be fixed now.

However I finally realized what is a general problem with mobile uploads. Many cellular providers do not send mails via their mail servers but rather connect your phone directly to our mail server. If you have a weak connection and upload a big photo - it may not pass through because connection closes preliminary due to some errors. Mail protocols are not as error-resistant as ordinary http.

So the general advice is:

If you can't upload a photo from your phone via e-mail, try reducing the photo. If your phone has some kind of simple photo-editing possibilities - use them, or try to shoot a new photo in lower quality/resolution.