It depends entirely on where you use it. On an intranet for example, you don't need to care about that.
And if by image activation you refer to captcha, they aren't entirely secure. Either due to lousy implementation or through image processing. Sure, even if not fool proff, they do make things harder. But they are also annoying from a user persepective.
As such, it should be used only if necessary. And if you do need it, you'd hardly ask for how to open an external mail client, but rather how to deal with sending emails server side.