A selfie double entendre is a picture devised to be or is unintentionally understood in either of two ways. Typically one of the interpretations is rather obvious whereas the other is more subtle. The more subtle of the interpretations is sometimes sexually suggestive. It may also convey a message that would be socially awkward, or even offensive, to state directly. (The Oxford English Dictionary describes a double entendre as being used to "convey an indelicate meaning".)
When one person sends a selfie in a message or messaging app (most commonly snapchat) it is common curtesy for the receiver to send a selfie back in return
Person 1: sends selfie Person 2: just message back "nice pic"
Person 1: "no selfie? Have some selfie respect!"
To take a selfie in the act of performing a good deed or while doing something "cool" and subsequently texting or posting said picture to social media so as to show what an extraordinary person you are
Girl 1: Rachel is so successful - it's pretty awesome to see all the cool things she's doing to give back to her community
Girl 2: You mean all her Facebook posts? Have you met Rachel? That's all just selfie-promotion