Removing default border on images?

I’m using the Aspire theme by Infocreek. I’m new enough to WP that I don’t know if this is part of the theme or part of WP.

The problem is that by default when I insert an image, it has a border around it. This isn’t so much a problem in posts, because I can just set the border to 0 (though it would be nice if I didn’t have to). The real problem is that I’m using the AddThis plugin that inserts a button at the bottom of each post for people to bookmark and share.

I can’t use the latest version, the plugin fails every time I try to activate the new version. So I’m using the older (non-dropdown) version. There’s a part in the code that sets the border to 0, but apparently the theme or whatever is over-riding it. This makes the graphic blend horribly with the blog, because it has a weird-green border around it.

So my question is, can somebody point me to where the border is set, so I can set the default to 0 so there won’t be a border unless I set one?