I have searched the navigator and found my way to “portfolio color box”.
However the images that I wish to change do not appear anywhere that I can see.
They are not loaded as images as such but rather they are are background image on a div container.
To change these… select the container Portfolio (as you already found) then go to Background in the designer (picture below)
Click “Portfolio-one.png” and you will see that you can replace the image from there.
I should have mentioned how to find it sorrry!
You also have a navigator across the bottom of the main window, if you click an element you can see all it’s parent elements and you’re able to click one there too. Although not useful if you’re looking for a child element.