How do I know how many megapixels have my photo?

Really easy, here's the formula:

(width in pixels x height in pixels) / 1.000.000

For example, an image with a size of 1.200 x 900 pixels has: (1200 x 900) / 1000000 = 1,08 Megapixels

What's the best format and color profile to upload my photos? What are the high-resolution and "retina" displays?
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