How do I remove the border around images?

[Karl Core]

I have no usability study at hand, just my own surfing experience.

*Your* surfing experience != everyone else's. In fact, as someone
intimately familiar with web development, your impressions of "ease-of-use"
are polluted.
Most people's surfing experience do not mirror our own.

In
several pages where clickable images have been made borderless, I
couldn't figure out easily which images are clickable.

And while a border would certainly made the distinction more clear, that
doesn't mean that a well-designed icon wouldn't have had the same
ease-of-use.
I would venture to guess that most users don't have the same familiarity
with the meaning of image borders that you and I have. If you're like me,
you've been using the web since the times of Mosaic 1, when all images that
were links had borders. These days though, most people's experience doesn't
span that far back and pretty much every navigable image hasn't had a
border.