Yes like Mariah said you have to only donate 1 but: To unlock the museum shop, you first have to donate at least 20 items to the museum. You also have to donate at least one item to each of the four categories: bugs, fish/deep sea creatures, fossils, works of art. After this, you will one day find Blathers thinking about something when you first go into the museum.
When you see him with a thinking icon above his head just go up and talk to him and it will be added to your town projects menu at the town hall.
Could you PLEASE have relevant titles in the future... They are supposed to represent what we can expect in the thread, and question marks could mean anything.
Anyway, as has already been answered, you need at least one painting.