Here's a short introduction to the chroma key effect. I have a dandelion picture which can be used as a good example. Just apply this same approach when you are using video.

 

Step 1

Open ZS4 and search in the file tab until you locate the picture you downloaded from here.

Now grab that green down pointing arrow and drag it into the track display area like so:


 

Step 2

Now click on the f/x tab in the track display treeview area, and open the keying filters folder (effects/video effects/pixel filters/keying filters). Click on yuv/chroma key and drag it onto the flower18.jpg track in the track display. (This sets the key effect as a child track of the flower picture)


 

Step 3

The display will switch to the current track tab once the effect has been added. This allows you to set the parameters for the effect.
Make sure that small toggle button with the green square on it is pushed in. This puts it in colour picker mode.

Select a large area representative of the background colours, as shown in the screenshot.


 

Step 4

The display automatically switches to the rectangle view at the top where you can see the results of the key in the grandparent track (MediaMixer rectangle at the left) and the vars tab in the treeview panel (lower left)
Since not all of the background was keyed out with the default settings, you may wish to adjust them now. 

In the vars tab, click to the right of the line (default setting) on the stripe labeled "y tolerance". Adjust it until all of the background is transparent but all of the flower is showing in the MediaMixer rectangle (top left).


 

Step 5

Now you may want to make the edges of the dandelion a bit smoother, so click on the f/x tab again and open the convolution filters folder. Take out a key smoother and drop it on the flower18.jpg track. It should show up above the yuv/chroma key in the track display. This is good.


 

Step 6

To adjust the key smoothing amount, you can change the matrix size and effect levels on the vars tab. I suggest clicking just a bit to the right of 0 on the matrix size strip. This will already do the trick for this picture.

Please note that making the matrix size larger will slow the program down exponentially, as with all matrix effects (convolution filters). A bit should do it.

To get a closer look at the result you can resize the panels, by dragging on the crossbars, to enlarge the MediaMixer rectangle.


 

Step 7

Now you can save a png image (with transparency) by pressing the export button on the flower18.jpg track (green circle with arrow) in the arrangement tab.  Use the browse button to choose a file name or type a path and file name into the file name text area.  Be sure to specify an extension of .png, otherwise a video will be generated (default behaviour).  Then press export.