How do I move photos from my Galaxy S3 to LG G4 via Picasa?

[AC Question]

Moving Photos from Galaxy S3 to LG G4 via Picasa

I have many photos and videos on an old galaxy S3. My picasa (and by extension, google plus) account are set to automatically upload photos / videos. When I take a photo with the phone, a few minutes later I can see it on google plus. I also transfer my photos from the phone to my computer using picasa.

I recently got an LG G4. I want to sell my Galaxy S3. What is the easiest way of taking any photos/videos that are not yet on my computer in picasa, and moving them to my computer? I would like to be able to wipe the S3 clean to sell, but I want to be absolutely certain all of my photos are saved to my computer first.

My second question is this: what is the best way, going forward with my LG G4, to keep plenty of space on my phone? With my Galaxy S3, I would connect to my computer as an external hard drive and manually move photos. This resulted in duplicate photos on my hard drive and google +. I think that if I instead move photos from LG G4 to computer using picasa, I will wind up with one copy on my computer and one on google +, but I want to confirm.