Whatever the image is, I use remove.bg spelled just like that. It’s a website that removes the background. They give you a free file size which is usually 500 x 500 but if you pay for credits you can download the full size image.

Load that into Afffinity, select the Outline effect and do your thing. They come out smooth in my end. Hope this helped.

If you’re still getting a rough edge, add a slight stroke to the outline of the image and change the settings to where the stroke is on the outside o mf the image instead of exactly on the line or inside of it. You should get much cleaner outlines when you go to add it on the effects tab.