In the other skins, the syntax [[File:example.jpg|border|caption]] displays a border around the image.
This is useful for image with a white color on the outside that needs to be "visible" (ex. Russian or Polish flag).

Currently Timeless does not display a border with this border option.

Suggestion of CSS to add to solve this :

.thumbborder {
	border: 1px solid #eee;
}