For Xubuntu versions before 11.10
Create a 96x96 jpg (JPEG) and renamed it as
For Xubuntu versions 11.10 and above
Create a 96x72 png (PNG) and renamed it as
Log-out to see your masterpiece.
The in-built image editor in Xubuntu is Gimp.
Load the image in Gimp and crop the image to approximately 96x96 or 96x72 pixels depending upon your xubuntu version as described above.
Then fine-tune your image via Image - Scale Image...
Break the scale link as shown and enter the pixel size
96 x 96 or
96 x 72 as appropriate. Complete the scale by clicking the Scale button
Save the file. If necessary save the file type when saving (jpg or png).