I don't have a picture, but it's easy.
Imagine you've taken the wheels off and are behind the wheel, looking at the back of the center cap. It has a few tabs that push outwards from the center, gripping the back of the wheel. Inside the center cap there's a groove that runs through all the tabs. There's a steel C-ring that lays inside the groove, pressing outwards. That way there's always pressure pressing the tabs towards the wheel.
Sorry, picture is worth a thousand words.
Long story short is that removal is a 30-second process once the wheel is off. Use a screwdriver or needle-nose pliers to pry the C-ring out from its grove. This is very easy. Then press the center cap. The tabs happily bend inwards (no risk of breaking, trust me) and the center cap pushes out.
Shorter: remove ring, push.
That's it.