Well, I've done some investigating and it seems that images that are sent in, can't be bigger than 1mb.
It accepts drawings that are 800 x 600 px in size but I feel that they are reduced to 400 x 300 automatically.
The only suggestion I can give you, is to make the printing much darker, so that people can read the text.
I'm sorry about that. I look at the bright side, at least you can send your drawings in! It's only a new feature so I'm sure it will improve with time.