For Affinity Photo's sub brushes, select the paint brush tool found in the tools bar. Go to the brushes panel, you can find this via the view menu and studio menu and then double click one of the presets or go to the right side menu and create a new one. In the editor panel you will notice there are four tabs, go to the fourth tab.
You will probably see an entry or two or more already. You can remove any of the entries by clicking the entry and clicking the remove.
You can also add a new sub brush to your current stroke by clicking the add bitmap option or add round. Click the add bitmap option and then browse for a file such as stroke PNG file but it can be anything such as a lot of text or a rough image or one of your photos etc. Click open and you will see the preset has been added. Select it and you can change how it draws with the 'at current nozzle' or behind or at end of stroke, the difference is often very subtle.
You can also blend or use the new design to erase which is actually quite effective in create some very unusual strokes as it cuts away from the current stroke. You can see the result in the preview top screen.
Some images such as standard square images are rarely that effective, I generally go for PNG files that have transparency such as a circular or arc design etc. You can create an image and save it as a PNG file and then use it as a resource in your strokes. Remember to change the spacing for your resource otherwise the end result can be much the same whatever image you use. You can add more and more to your stroke and you can build up a complex design with blending and erase (not sure if there is a limit to the number you can add but the more you add, potentially will make the stroke harder to process and harder to work out what will be achievable)
You can edit the sub brush to change the spacing etc as well as the size and dynamics etc and this is for each resource added. Simply select the resource and then click edit and you will see three tabs this time)
Close the panel and then use your new stroke. It should be noted that if you want to keep both the old one along with the new, it is best to duplicate the resource before you start changing the settings.
You can also export them via the right side menu of the brushes panel and save the resources for future uses and projects as well as to give to your colleagues etc