This is going to sound self serving, so I apologize in advance. Looking at your photos, the chain looks awfully clean, and I see the beginnings of rust on the edges of the rollers. Not necessarily a terrible thing, but it makes me wonder about your chain's lubrication.
Depending on your lube, water can enter the chain carrying fine silt. When the water evaporates or is slung out, it leaves the silt deposited between the plates, making the chain stiffer. That causes the chain not to wind onto the smaller cogs properly, making the chain climb up onto the tops of the teeth and skipping.
Bringing the RD up and forward to move the jockey wheel closer, and/or increasing chain wrap will help, but the right answer might just to clean and lube the chain properly to maintain maximum suppleness.
Update: Moving the derailleur closer to the cassette as tonyxcom suggested made a SIGNIFICANT Improvement! It only skipped 2 or 3 times on my commute this morning, and only in 7th gear. A little more fine tuning should wrap this one up.
I'll also get some chain lube as FBinNY suggested.
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