You can't. [IMG] is a kind of Discuz code, because most of the rest are typesetting in format and font style, so it belongs to the same category. IMG quotes pictures, which may lead to uncoordinated styles or other situations that affect the layout, so the permissions are listed separately. If you can't use IMG in Discuz code, you can also use html, but generally 99.99% of forums have closed the opening of html to ordinary members, so if you can't use IMG, you can't basically map it in the signature.