I never know a) how much to compress the file, or b) how many pixels the image should measure, in order to maximise quality but minimise loading times. I just guess, and I have no idea how good my choice is.
If you look on the file info, it should say how many mb the image is.
Something like this below, whether on Windows, phone, ipad. There'll be a drop down menu which tells you how big the file is. You can get free, quick and easy file reducing software too.
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Pretty much everything else, I love.