i beleive thats true

Raydog joined in 2015. Imgflip was first created sometime around 2011 or something.

then who was first to join?

I have no idea. He joined on March 28, 2015. Below, I have listed a bunch of users who definitely joined before him.
imgflip.com/user/PraytheReyAway
imgflip.com/user/Beckett347
imgflip.com/user/Invicta103
imgflip.com/user/hokeewolf
imgflip.com/user/platypus_hex
imgflip.com/user/Steamfitter602
imgflip.com/user/discolu
imgflip.com/user/ChumpChange
imgflip.com/user/IWantToBeBacon.com
imgflip.com/user/JaiploSueisnuaX
imgflip.com/user/The_Lone_Haranguer
imgflip.com/user/liarspew
imgflip.com/user/Montresor_SP
imgflip.com/user/catfish94
imgflip.com/user/tipsy_taylor
imgflip.com/user/civor
imgflip.com/user/luftweg
imgflip.com/user/regularfeller
imgflip.com/user/GAME_KING
imgflip.com/user/SeanBeanDip
imgflip.com/user/The_Psychedelic_Fur (?) He joined the same day as Raydog, so maybe
imgflip.com/user/whistlelock
imgflip.com/user/TheRaven8386
imgflip.com/user/ekult
imgflip.com/user/Mig_L
imgflip.com/user/Kanaloa
imgflip.com/user/InfernoKid-Sandstorm
imgflip.com/user/jefftims
imgflip.com/user/LadyDeerHeart

And lots more!

And here's another one: imgflip.com/user/acuity12