The answer depends on the "decentralized network", we have reddit clones and we have crypto-currencies as two extremes.

Based on this site I suspect that the question is about payment-focused crypto currencies and there the answer is simply that they have considered this and avoided this in the design.

See, the payment networks focus on having only validated transactions that move some sort of funds. They don't allow just random content to be stored on the network, only transactions. Some exceptions are Bitcoin based systems that a comment field can be added, and this naturally is a great thing to focus on and put data in that might fit your definition.

So while nothing but actual transactions can be stored, some type of free-form data is available in the shape of a comment field (Bitcoin and Bitcoin Cash: an OP_RETURN field).

The solution to avoid any questionable content is to severaly limit the size of this field. As any image is going to be at minimum a couple of kilobytes or half a megabyte in size, the comment field is limited to less than a quarter kilobyte. A magnitude or two too small for most obvious questionable content.

This is the way to avoid this issue, and after more than 10 years of operation this has proven to be successful.

Corrupt people always seek to pollute everything that exists and get things illegally and they always develop their evil methods. There is no doubt that pollution will be done at some time, but it will be countered at the same time

how do you deal with the inevitable contamination; what measures you put into place?
there will always be contamination.