Simple image editor tool to draw a simple box/rectangle in an existing image

If you're using a GNOME environment, try gpaint. It's in the universe repository:

If you're using a qt-based system such as Kubuntu or Lubuntu 18.10, try kolourpaint. It's in the universe repository:

If you want something a little advanced, try ksnip. Its homepage is here.

Various versions and package formats of ksnip are available from here as

• an AppImage. You can see What is an "AppImage"? How do I install it? for more on AppImages.

• a .deb and

• a snap package

(You need to expand the  Assets section below the relevant release to get the download links.)

When I need to edit a picture quickly, I do it through the web. Simply because it starts much faster than any "installed" app I ever tried :/

I use www.Photopea.com. To draw a rectangle, choose a Rectangle tool in the toolbar, set the Stroke to Red (or any other color) and Fill to None.

I think it is better than Pixlr, it doesn't require Flash and supports more input / output formats.

It also has advanced features. If you ever need to do more, it is easier to extend the knowledge of one advanced tool, than to learn 37 new single-purpose tools.