How can I generate the two images requiered by <webaudio-slider> with the Knob Gallery

Every time I choose a slider in the knob gallery, and click on "easy rendering", the generated image contains sprites and seems to be usable with <webaudio-knob> not with <webaudio-slider>. Can the knob gallery and the rendering tool be used for generating the two distinctive images compatible with <webaudio-slider> ?

Thanks in advance !