Hello nekonog!
You may set border around your image doing any of this:
1) select your image, open Properties window, at the right corner you'll see Border, set any size (5 for a example).
2) open CSS Styles window, create new style (click on Plus + sign), select Border, set border width and color and select Solid from the drop down menu. Select your image, apply your new created CSS to it (click on the name of new CSS Style at the CSS Styles window).
I would recommend to use both things together, because first will work in all browsers (but the border will always be black) and second will let you set the color.
Good Luck!