n case you are not aware, GIMP is an open source image editing software that is bundled in most Linux distro and is touted as the closest alternative to Photoshop. It comes with plenty of toolset and filters for you to create/edit your image, and the best thing of all, it is free.
However, as good as it can get, there are several handy and useful features Photoshop that are not available in GIMP and one of them is the Save For Web feature. I used the Photoshop’s Save For Web feature extensively as it enables me to optimize my screenshots to the smallest possible size without affecting their quality to a great extent.
Luckily, GIMP supports plugins and there is this plugin available for this purpose.
If you are using Ubuntu (or any other debian-based distros), you can easily install the Save For Web plugin via the following command:
sudo apt-get install gimp-plugin-registry