The poster needs to have all original images created by yourself. It should fit your film's story, your film's genre, and be recognisable as a film poster.
Include title, stars, credit block, main character(s).
Industry standard is 30x40 inches, quad size, and you should (unless advised otherwise) stick to this sizing ratio, but make it smaller.
Create a document 15x20 inches, 300 dpi.
When it's finished it needs to be a jpg uploaded to the blog.
You may like to see how it looks in a real venue.
