It's always about incentives. Node runners are still free to determine what's get filtered. Miners are free to determine which transaction they mine. If enough node runners are fed up with a bloated mempool on their node, they should take action and this in turn will incentivize miners to consider the risk of slower block propagation. Lynn's point does raise a valid about the cost of transactions that need to disincentivize jpeg's over valid tx's. In times with low tx volumes, jpeg bloat provides extra income stream for miners. While in times of high tx volumes, jpeg bloaters will think twice if the cost of their monkey pics are worth it.

Your mempool is a fixed size and your node drops transactions from mempool based on fees keeping only the highest fees that fit into the fixed size. The block size and 10 minute target time also don't change. That means no larger mempool or node resources needed for a stream of full blocks of JPGs than a stream of full blocks of monetary transactions.