In any case, one of the two ways requires a plugin from the registry, while the other one is a bit more manual but nothing too time-consuming. So we shall start with that so you can decide which one you prefer.
With No Plugin
Let’s say you have a raw screenshot straight from the source. A lot of the time, the resulting screenshot can turn out huge in file size. Just take Windows Vista and 7’s Snipping Tool. It creates high-quality screenshots, but in no way optimizes when you’re saving the file. Compared to the screenshot saved with my preferred screencapture utility, FastStone Capture 5.3, the Snipping Tool’s file was almost 4 times as large!
So when you obtain an image file like that, drag it from Windows Explorer to the GIMP window, head to Image > Mode > Indexed.
You can type a value in the box for maximum number of colors or use a web-optimized palette, which will make your image suffer heavy quality loss.
You can always try the other mode using Ctrl + Z to undo. Now go up to File > Save As.
You’ll want to name your file with PNG as the file extension as it’s a screenshot. In the next dialog, uncheck Save Resolution, Save Creation Time, as well as the comments, and then hit OK.
The results when I optimized the 1.17MB file ended up having 481KB, which isn’t too bad!
Alternatively, you can also save the image as a JPG file.
Just make sure you check the box to “Show the Preview in Image Window“, as it will display the potential file size in the same dialog while you can still monitor the quality of the image.
In this case, the resulting file turned out to be 458KB, which is still smaller than the original one.
Save For Web is a plugin from the registry that will create an option to optimize the image for web use under the File menu. Download either the zip or the tarball file from the site. You need the exe file in the contents of the zipped file in your GIMP plugins folder. If you don’t know where your plugins folder is, you can find the location by going to Edit > Preferences in GIMP.
Now expand the Folders option and highlight Plug-ins.
That’s the folder you want to copy the exe file (webexport.exe) to.
Now restart GIMP (remember to save your work) and you’ll notice the new option under the File menu.
When you’ve got an enormous file, you can use this option to preview the file size according to the file type you’re interested in saving.
You can already preview the file size natively in GIMP when dealing with JPG files, but this plugin gives you the same option for PNG files, which are most likely what you want for your screenshots. Using this plugin, I was able to get a 186KB PNG file with 8 bits per pixel, which is great compared to the 400+ KB (24-bit) file using no plugins like detailed above. The quality was virtually the same, as far as I could tell.
I haven’t tried this, but there’s apparently another plugin, RIOT (Radical Image Optimization Tool), which IrfanView uses to optimize images.
What do you use to optimize screenshots?