No apologies necessary. This is what the forum is for 🙂
4:3 is fine for the aspect ratio if you want your photos to fill the frame. YouTube will add black bars on the side when you upload it to make it fit their 16:9 player. Other than being standard definition, I’m not sure why your export would look extra pixelated. Your Pixel Aspect Ratio (PAR) should probably be set to “Square” instead of “D1/DV NTSC (0.9091)”. This isn’t going to DVD so there is no reason to use non-square pixels.
I personally would recommend creating a HD 1280×720 sequence to work with. Since you say only a few of the photos are the 720×480, you can either leave these at that size and put a background behind them or try scaling them up and seeing how they look. Yes, there will be empty space on either size of your photo while you edit, but if YouTube is going to add pillar boxes (black bars) anyway, I think you are better off starting at a higher resolution.