|Common Blue butterfly, Polyommatus icarus, and Brown Argus, Aricia agestis.|
There are two features of the underwings that are easy to see in photos; rather less so on live and active creatures.
The Brown Argus has no spots on the lower half of the underside of its forewings. The Common Blue does.
The Brown Argus has two spots on the leading edge of the underside of its hind wings that are aligned together, like a colon. On the Common Blue, the equivalent spots are aligned so as to form part of an arch of spots following the line of the edge of the wing.
So now you can easily tell which of these two is which, yes?