External image vulnerabilities

Question

What security holes can appear on my site by including external images via img tag and how to avoid them?

I\'m currently only checking the extension an

Answer 1

The primary holes that can be exposed are those where corrupted images cause buffer overflows within the browser, allowing arbitrary code execution.

If you're only putting the images into an tag there shoudln't be any vulnerabilities relating to sending alternative MIME types, but never underestimate the stupidity of some web browser developers...