An issue with security images?

You know those little boxes that say, "Type image below", and then they have a scrambled mixture of letters and numbers, kind of like this: "KU6FRG"?

Well, whenever I type these into the box provided, the numbers do not register, and the website will return a message saying, "The letters in the image do not match. Try again."

Why isn't my answer registering as correct, even though I check it like ten times to make sure it is absolutely, character-for-character, no different than the image on the screen? Is there some kind of setting on my computer that is faulty and that I can change?

This is really annoying, so any help you could provide would be really appreciated. Thanks in advance. :)