MySQL外键允许NULL吗?

I'm piecing together an image website. The basic schema's pretty simple MySQL, but I'm having some trouble trying to represent possible admin flags associated with an image ("inappropriate", "copyrighted", etc.). My current notion is as follows:

tblImages (
    imageID INT UNSIGNED NOT NULL AUTO_INCREMENT,
    ...
);

tblImageFlags (
    imageFlagID INT UNSIGNED NOT NULL AUTO_INCREMENT,
    imageID INT UNSIGNED NOT NULL,
    flagTypeID INT UNSIGNED NOT NULL,
    resolutionTypeID INT UNSIGNED NOT NULL,
    ...
);

luResolutionTypes (
    resolutionTypeID INT UNSIGNED NOT NULL AUTO_INCREMENT,
    resolutionType VARCHAR(63) NOT NULL,
    ...
);

(为了便于阅读而被截断；我发誓，各种外键和索引是有序的)