First, download the latest software version (1.32 for Windows).

Then install it in the default directories..

When you open up the application, you get a minimal window.

XnView comes with a simple capture feature

You can set the captured window, as well as the delay, and select "client area"

Here, I am capturing part of a Maple session...

The captured window is then brought into XnView for editing.

With the left mouse, you can select an area for "cropping".

The size of the image is reduced. Note that the image is saved in rgb (24 bit format).

Now, save the image in gif format.

With 256 (8bit) color.

Now, open up the saved (gif) image, and look at the "index" of the background color.

Now, save the file again, as a gif file, but choose the options, and set the transparency color to be the index of the background color. I have saved tghe file as "trans-maple.gif".

To test the transparency, create a little html code to display the gif against a gray background.

As you can see, it works, with pleasing effect