How can I use the CSS before selector for links except those containing an

How can I use the CSS before selector for links except those containing an

a:before {
content: "› ";
}
a img:before {
content: ""
}
I know the above syntax is incorrect technically and semantically, but I
want to exclude the content from the a tag any time an img is included
within. Is this possible, or should I do it with a class instead, i.e.
a:before {
content: "> ";
}
a.no-before:before {
content: "";
}
I'd prefer to do it without having to define classes for it, but I can see
that might be required.