CallManager Administrator and OS Administrator access
Open the picture with The Gimp
In my case, the image I selected is 225x168px, so it doesn't meet the size requirements (see Prerequisites).
Go to Image > Canvas Size (If your image is bigger than the specified size, go to Image > Scale Image and follow next steps).
Click the chain icon next to the pixels drop-down menu, and change the canvas size (or image size if you are scaling it) to Width:320 / Height:212
Click the "Center" button so that the image stays centered on the new space, or drag the preview thumbnail to a corner of your choice within the canvas' space (if you are scaling the image the "Center" button and the drag option are not there)
Note the changes
Now we need to put a white background.
Right click the image layer on the layer window
Click Flatten Image at the very bottom.
Now we need to make sure there are no more colors than the phone can accept.
Go to Colors > Posterize. This tools works with powers of 2. An IP Phone cannot display more than 65535 colors, so in the increase or decrease the posterize levels to no more than 15, as 15 is the closest power of 2 to 65535.
Go to File > Save as > choose PNG as the file type
Now lets fabricate the thumbnail
Go to Image > Scale Image and type 80 in the Width field
The Height field should automatically populate to 53
Save the image in PNG type just as we did before
Try to keep simple, short names like img.png and img-tmb.png
Now we will tailor the XML file that lists the available images
Copy this content to a regular text file, and save it as a .xml archive: