Actually the default style of the img tag is defined as to fill the background with #f4f4f4 color which appears as the border in your pictures. You either need to make a CSS upgrade inorder to edit it and make it change to any other colour or transparent. Else you need to all this style manually in each img tag to override the CSS settings. To do it you need to add the following with the img tag.
<img style="background: transparent;" src="link_to_image" />
the style=”background: transparent;” will override the colour border settings and remove the border from your images.