How do I create a suitable custom icon with Windows tools?

Your image will look its best on all 3Dconnexion (on-screen) displays, if it is 24 x 24px in size and has a transparent background.

For an optimal result, you should therefore use a professional image editing program to create a custom icon.

However, a good result can also be achieved with Windows tools. All you need to do is:

Take a preferably square screenshot (Windows key + Shift + S) of the icon you want to use in your Macro/Radial menu.
Use CTRL + V to paste the screenshot into a new Microsoft Word document. Then select the screenshot and click on the rightmost tab “Picture Format”.
Now click on “Color” and select the function “Set Transparent Color”. With the pencil that appears, click on the background of the screenshot to make it disappear.
By right-clicking on the picture, you can select the “Save as Picture…” option. Now save the picture in PNG format.
Next, open the image you just created in Windows Photos and click on “Crop”. On the right side under “Aspect ratio” select “Square”, drag your image to the center and click on “Save a copy”.
To resize the image to 24 x 24px click on the three dots in the upper right corner and select “Resize”. Then click on “Define custom dimensions” and enter 24 pixels for the width. The height should also automatically adjust to 24px if the “Maintain aspect ratio” option is selected. Click on “Save resized copy” to save your custom icon.
Done! You can now load your custom icon into the driver and assign it to a macro/radial menu.