The portfolio is a grid of photos of work you have done. It is one of the items the action centre nags you about, because a profile with no pictures converts badly.
Open Portfolio
Profile, My business, Portfolio. Your images show as a grid.
Upload an image
Add a new photo and it appears in the grid.
Remove one
Deleting an image takes it out of the grid.
Save your changes
Save to keep the grid as you have arranged it.
Check it from the customer's side
The portfolio appears on your public profile under the details tab, where a customer can tap any image to open it full screen.