Of course everybody knows the famous picture from the Tiananmen Square Massacre and is considered as one of the most powerful picture of protest.
The picture from Portland has so much symbolism covered.
What I can see is
Vulnerability The naked body shows signs of weakness and can easily be wounded. The body may seem exploitable. The contrast is the riot gear of the police. Heavily protected and armed.
Openness When you are naked you may not just feel exposed, but also free and open. There is nothing physically to hide anymore. The contrast is the anonymity of the policy. Masks and Helmets and complete protection of the body.
Individuality Each human is different. We put cloth on to symbolize that we may belong to a group or have a certain opinion on something. The naked body shows us only the individual. It may show us age and maybe how we treat our bodies through food, fitness and so. Yet it shows us in which group we all belong. We are human. The contrast again is the anonymity of the group military police style clothing. You may still see differences like in height or bodyweight, but the group will always have the picture of power, unity and order.
Motherhood The most fascinating facet here for me is the symbol of motherhood. The female body. The source of life. The opposite party can find themselves subconsciously in that picture and where they came from.
I have not seen so portray of passive disarming power. At least in the picture there is no really provocation. For me this will be pretty much the Picture of Year at least.