I'm torn between two ideas concerning this picture. One is what was discussed previously -that titans are people who've some way came back from death, and that's way that guy and titan share the big bushy beard. However, their faces aren't alike, and we don't have much proof that this random guy has become a kyojin.
The other one is a bit silly, but I wanted to know whatya think about it. What if titan's aren't so dumb and they actually shave their facial hair? That would explain how the bearded titan is the only one with beard.
The bearded guy and Titan looked even more similar in the Manga. It's not the only case either where Titans with unique plot relevance look like people who had recently died/been missing.
I don't know if you're up to date with the Manga or not? If not then this would be a spoiler
Longer beard makes sense if gramps survived a couple months before becomming titan snack.
I honestly think that all titans are originally human, the ones that are mindless are the ones that have no control over their titan form and become a "full fledged titan" thus changing species.
I mean isn't it ironic that the weakness of a titan is the nape of it's neck?
Titan shifters reside in the nape of it's transformed titan state.
Honestly in my opinion I think it's full fledged mindless titans were just manufactured by the ape titan, since titan shifting are able to transform into a titan body out of nowhere ( looks like it uses lots of heat+ sun + air i'm guessing), all titans are indeed humans.
Current humans are unable to transform into titans unless they have "titan blood" or whatever they call it ( eren was injected by his dad with some sort of serum that allowed him to transform into a titan).
I don't think titans can "infect" humans to transform into titans, I think the ape titan manufactures them as much as he can but humans are dead like dead no way they can come back to life unless they had "titan blood" in them.
From reading the hints that the manga gives you, I just feel that the creator is being very subtle about it.