Tool to add shapes, annotations and text using templates to images

Warning: broken. Shutter is severely broken in Ubuntu 18.04 and not available in later versions.

Shutter (which you can install from the Ubuntu Software Centre or sudo apt-get install shutter) is a tool which has a variety of options for taking and annotating screenshots. (Note: You can annotate any images of your choice, not just screenshots.)

Flameshot

It comes with some cool annotation features like Freehand drawing, Lines, Arrows, Boxes, Circles, Highlighting, Blur, etc. It also comes with many on-screen buttons as well lots of customization options.

Its written with QT/C++ and it's very easy to install in Ubuntu(18.04+).

apt install flameshot

You can also try to compile for older versions.

I would use Inkscape

(which you can install from the Ubuntu Software Centre or sudo apt-get install inkscape).

You will need to right click the image, select Open With -> Other Application... and chose Inkscape from the list. After you have done this the first time, you can just right click -> Open With -> Inkscape.

This will import the image into Inkscape and the page will be sized to fit the image. You can then make your annotations.

To save it, you need to use File -> Export Bitmap, click Browse... and chose your original image.