Save for Web is a very useful feature built into Adobe Photoshop. To bring the same functionality to GIMP, there is a Save for Web plugin for GIMP too. Like most other plugins, you would find ready-to-use versions for Windows; but Linux users are usually forced to compile the code.
If you are on Windows, follow the tutorial Install Save for Web plugin in Windows instead.
In this short tutorial, you will learn to install the Save for Web plugin in Ubuntu Linux – without compiling code or any other complex processes.
Download the Ubuntu Deb archive of Save for Web plugin to your desktop.
Extract the downloaded archive folder.
Open the extracted folder. Inside it, you will find an archive folder, named data.tar.gz. Double click it. Then, double-click usr, lib, gimp, 2.0 and plug-ins folder. In it, you will find a single file, named webexport. Right-click it and click Extract. Extract the file to Desktop.
Open a terminal (Applications > Accessories > Terminal). Type sudo nautilus /./usr/lib/gimp/2.0/plug-ins/ and click enter. Enter your password when asked.
Copy the plugin file webexport from Desktop to the plugins window that you just opened up.
Restart GIMP if you had it open. In GIMP, you can access the Save for Web function from File menu.