Moving photos from PC to iPhone?

I wanted to move photos from my Camera Roll to a different album on my iPhone (not in the Camera Roll, so it wouldn't take up iCloud space, etc).

I transferred the photos to my PC by just dragging and dropping from the DCIM folder, but now I want to transfer them back to the iPhone (but not in the Camera Roll).

I don't want to use iTunes' photo sync tool to move the photos from my PC to the iPhone, because I have several gigabytes of other photos on my iPhone already that would be deleted if I sync'd my PC photos to the iPhone.

Anyone know an easy (free) way to do this?

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Alternatively, how do I get the several gigabytes of photos on my iPhone that are NOT in the Camera Roll onto my PC so I can sync everything?