To answer your question, I think 7-8am so people can check it before work is good, lunchtime is good, and maybe 2-4pm and onwards would also be a good time for people coming home from school etc. In general, daytime seems to be better than having your them come out in the middle of the night (I think). I'm using Eastern Standard Time.
But I feel like I should also tell you that I didn't get to where I am on imgflip by submitting my memes at certain times of day.
What helped me most was not focusing on my memes so much, but just making funny comments on memes on the front page. That's where the views are anyway. (The idea is instead of trying to wait for people to come to your memes, you go to where the people already are and put meme comments there.) That's what worked for me, anyway. :)