Whenever you take screenshots using the default screenshot tool of Ubuntu, the image does not look as good as the screen itself (the quality is deteriorated). So if would like to take high quality screenshots, you have to install an advanced tool. Our suggestion would be to use the screen capture tool Shutter. Shutter is a feature-rich screenshot program for Linux based operating systems such as Ubuntu.
You can take a screenshot of a specific area, window, your whole screen, or even of a website – apply different effects to it, draw on it to highlight points, and then upload to an image hosting site, all within one window. Shutter is free, open-source, and licensed under GPL v3. As you will see in the preferences, you can define both the image quality and the file format to be used. This gives you plenty of flexibility to achieve the desired image quality. Within the settings, you also have the possibility to set Shutter as your the default screenshot application.