Lately, I wanted to put rounded corners onto an image. I couldn’t do this easily with several of my usual tools for images (Greenshot, Paint.Net, IrfanView).

To help came a free online image editor. Many languages can be selected, including german, english, spanish, bulgarian, japanese.
Rounding corners is simple. You can select the radius of the corner in pixels, the thickness of the border and the color of the border.