When I publish apps I use the following screenshot sizes:
Phone: 1080 x 1920 I prepare 8 images with title, some fancy background and a screenshot inside a smartphone mockup. So it’s more than a simple screenshot. It gives some nice branding and helps you to stand out from other apps out there.
Tablet 7″: 1200 x 1920 – I do actually a couple of raw screenshots of 7″ emulator so that the user could know how the layout will appear on his device. No fancy design with titles etc.
Tablet 10″: 1800 x 2560 – same thing here, just a couple of raw screenshots.
all in .png format.
Hope this helps.