Removing Borders on All Images

it looks like the image class has a border attribute Try adding:
.aligncenter {border:0;}
to your Stylesheet. You may also have to add that to right and/or left aligned images as well.

Tried adding them and hasn’t work.

It’s definitely on the img class. Look in your style sheet for img:

Or put at the very bottom of style sheet:

img{border: none !important;}

The images aligned left and right work fine, center still has border.

CSS at the start of Images

img {
max-width: 98%;
}

.post-format-image {
background: none;
display: block;
float: left;
border: 0;
margin: 0 10px 10px 0;
}

.page .post-format-image {
display: none;
border: 0;
}

Here is the CSS for center

img.centered,
.aligncenter {
border:0;
display: block;
margin: 0 auto 10px;
}

img.centered must be picking it up somewhere else. Try this:

img.centered,
.aligncenter {
border:none !important;
display: block;
margin: 0 auto 10px;
}

Also, in firefox you can identify styles and try editing css using WebDeveloper toolbar (and add on) or Firebug