There are lots good answers already in this thread. I would say: "I vaguely remember that the guy in this picture is your ex."

"I kind of remember" that you mentioned is good. "I remember slightly" would be fine if you made a small change - "I remember him slightly". "I remember blurry" doesn't work at all". "I sort of remember" is another one that works. It's very similar to your first (I kind of remember")

Echoing the first answer: Either "I kind of remember that.. " or "I sort of remember that..." are both fine, very idiomatic, native speakers use exactly these expressions. "I remember him slightly." "I can remember him, but only slightly." This is a bit of a stretch perhaps. If you want to use "blurry" your sentences will be more complicated. But more colourful and advanced and metaphoric, if that's what you're after. "I have a blurry recollection of the guy in the picture being your ex." "I have a vague recollection, but my memory is blurry." "I can picture him a bit, but my picture of him is blurry." Memories can be blurry, or possibly your memory is blurry. Literally blurry is for pictures/images.

If I remember/recall correctly...