how to reduce size of an image from 31.1 kb to between 15kb - 20 kb

Use Gimp's 'Save for Web' plugin. You can download and install it manually, or just add the gimp-plugin-registry package from the Software Center which includes it (and many more).

Once installed, you just open the image in Gimp, select File -> Save for Web then play with the output settings until you have what you want. There are two tabs on the left. One is for image quality, one is for image size. You can even output the image in a different image format. As you make changes, the output file size (shown below the picture) updates. I use this plugin constantly and highly recommend it.

For me, the only problem is that with very large images it sometimes won't launch. If I resize down from say 100MB to a 10MB image size, it works fine.

There are several solution in here using both GUI and command line utilities.

For example:

aneeshep suggested to use

convert  -resize 50% source.png dest.jpg and 

convert  -resize 1024X768  source.png dest.jpg

Or even use the GIMP's built-it abilities suggested by tommy-brunn:

GIMP is probably the easiest way, since it has a fairly simple UI for such common tasks. All you have to do is open up your image and go to Image → Image Size and then change accordingly. There are ways to do batch resizing using the GIMP as well, but I don't know them by heart.

I am using the following packages for Nautilus:

nautilus-image-converter - nautilus extension to mass resize or rotate images
nautilus-image-manipulator - Resize and send images from Nautilus

You can install them with:

sudo apt-get install nautilus-image-*

After that restart Nautilus even by killing it (killall -9 nautilus) or by rebooting/logging out and back in.

You will have a context menu by right-clicking the images. You can rotate and resize the images in the simplest way.

To decrease image sizes in GIMP we have several approaches. Use a combination of all for best results:

• Crop image
  Usually there are parts of an image we do not want to keep in the final published version. Start with cropping your image to the desired content. Crop to selection is quite easy to perform as it allows you to select the interesting area prior to crop. This also includes a fixed geometry we may want to have.

• Scale Image
  Scanned images or photos of a modern digital camera are usually way to large to fit on a standard screen. We may want to scale the image down to the desired final size.

• Despeckle
  This plugin (accessed with Filters --> Enhance --> Despeckle) does not only allow to remove speckles or artifacts from scanned images. It also helps to smooth the image which may lead to a dramatic improvement in efficiancy of a later compression.

• Export
  In the "Export" dialog we can export an image to any compressed format by file extension. For JPEG compression choose filename.jpg. There we can also adjust the resulting image quality, with less quality on smaller sizes. For JPEG the optimum quality would be at about 90% but we can compress down to less than 70% without over obvious artifacts. This also depends on the image content. Use the preview option to control artifacts and choose a lower quality unitl the resulting image size is small enough but still has not too many artifacts.

Since you already have GIMP installed and running, you do not need to install any additional software.

The easiest solution would be to adjust the compression settings when saving as JPEG - moving the slider to 95-100% will somewhat increase file size compared to the default settings.

For image editing and re sizing best software is GIMP image editor .Install this using Software Manager.

To reduce Image size: Step 1:Right click on the image and open as GIMP image editor.

Step 2:Now Check the footer of the image and reduce the PX percentage (as per requirement)

Step 3: Now Click to file and export the image.

Step 4: Finally check the image size.

This is the easiest way to reduce image size.If you have any more doubt please feel free to ask.Thanks..