A husband and wife are not allowed to show chiba in public. That would presumably exclude holding hands in public. Posting pictures online sounds like public.
Just because someone doesn’t keep one halacha, I don’t know if I would call them “not frum”.
However, the issue of holding hands at the wedding is more complicated. Actually, many chassidim have a minhag that the chosson and kalla specifically do hold hands coming from the chuppaedited.
So it seems that if you do it, you at least have who to rely on- even if following your background you really should not be relying on those opinions.