When we create images using the gimp, the default format in which the images get saved are .xcf. eXperimental Computing Facility.
The .xcf files, though they have images, are not visible as thumbnails and are shown as below even in the thumbnail view.
To be able to view the .xcf files in thumbnail view properly in gnome, we need to install the package
gnome-xcf-thumbnailer i.e run the command
sudo apt-get install gnome-xcf-thumbnailer
or search for the package in the the synaptic package manager and install te same.
Once the package gets installed, log out of the system and login. Now open the folder that has the xcf file and we should be able to view the thumbnail of the xcf file as shown below.