What devise takes best pictures for ebay? 800 pixels,will give full stars?

I am realising a lot of people when shopping on line are useing there mobile phone or ipad etc… And i know that if you dont upload the correct kinda photos then your picture will not be zoomable ie which is needed for someone viewing on a mobile for instance

ebay advice that as long as your pics are 800x800 pixels or at least 800 on the longest side then they will be zoomable…
I took pics on my ipad but when i transfer them to my lap top to upload theyre not 800…
So does anyone know a device or way of makeing them 800 etc so they will be zoomable

many thanks in advance!

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