Alex,

The text above the password box reads, "If you'd like to have the option of deleting your post later, please provide a password (CASE SENSITIVE!):"

It has nothing to do with posting a photo.

What is it you are trying to use as a password? I'm not 100% sure what is allowable and what is not, but it may just be your choice.

Do this, try replying to this post and type in something simple for a password in the box and see what happens.

Greg