But wait, there's more?! Assuming the original picture had a color depth of 16 million colors, then the file size for that picture is (more or less) 24 bits per pixel. 15 000 000 000 pixels x 24 bpp / 8 bits per byte / 1024 kb per byte / 1024 mb per kb / 1024 gb per mb = 41.91 Gb file size!

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actually its 1024 bytes per kbyte / 1024kbyte per mbyte / 1024mbyte per gbyte, but basicly the idea was correct, just reversed.
(((((15,000,000,000 * 24)/8)/1024)/1024)/1024)= 41.91ish
Sorry, just saw the reversed to wrote it out.