Selections from brush strokes are easy to create in Affinity Photo and you can use them to make unique masks etc and they can then be used as a source for quick image brush strokes as well.
Make a new layer on top of the image and then go to the brushes panel and choose a stroke from the range of oils, pencil etc. Go to the tools panel and choose a paint tool and apply the stroke to the new layer and then once you are happy with your design go to the select menu and 'selection from layer and delete' command.
You now can delete the layer completely via the layers panel (as there maybe some leftover of the layer and you don't want to be accidentally using that in your image.
Make certain that you are on the layer you want to work with such as the background image. You can now copy and paste the existing image and generate a layer and duplicate etc
Or you can use the result with the brushes panel and right side menu command and new command as described above. The stroke is then saved to the panel and you can now use that as a wonderful image stroke and apply across the document such as changing the dynamics (rotation, scattering etc). If you used an oil paint stroke then you will now have an amazing new oil stroke design to apply to your images.