Well you could just put a comment on one of their memes about it. Although I would avoid putting a comment on a recent one, or on a meme they have in the top couple of pages of Hot (because if you do it that way, it looks like you're trying to use their position to draw people to your memes, which isn't great). So I would just scroll back a few pages in their profile and put it on one of their old memes, maybe.
Or, there's a way you can send a relatively "private" message to someone on imgflip. It involves writing a comment on the image page of one of their comment images they haven't submitted yet. If you want to go that way, then here's how you do that:
Step 1. Find a meme comment they've posted on one of their images, or someone else's. Right-click the image as if to save it to your device. At this point, the device should give it a default save name (something like "2h6kb2", for example).
Step 2. Type that code into your browser after "
imgflip.com/i/" Then you can write a comment on that image that the user will see in their Notifications, but it will be harder for other users to see.
Hopefully that's not too confusing.