The point that Karthikeya is not widely worshipped may be true in North India. But in Tamilnadu, he is very popular. This may be due to the fact that all the major incidents which involved Karthikeya took place in Tamilnadu. The Six Abodes of Murugan signifies such incidents. Apart from the Six main temples, there are numerous other notable temples dedicated to Karthikeya. Maruthamalai, Kathirkamam, Sikkal, Kunrakkudi are some of them. In Tamilnadu, he is regarded as the Tamil Kadavul (meaning Tamil God).
Outside Tamilnadu, Karthikeya is worshipped in Karnataka and Kerala also. Kukke Subramanya Temple is located in Dakshina Kannada district of Karnataka. Keezhattingal Sree Subrahmanyan swamy temple, Chacka BalaSubrahmanya swamy temple, Edappazhinji Subrahmanya swamy temple are a few of the many Karthikeya temples located in Kerala.
Outside India, Karthikeya is worshipped in Sri Lanka and Malaysia too. Nallur Murugan temple and Thirukkovil Murugan temple (it was affected by 2004 Tsunami) are notable temples in Sri Lanka. The Batu Malai Murugan temple is a famous temple located near Kaula Lumpur in Malaysia.
It is true that Karthikeya worship is not popular in North India. During the Gupta rule, Karthikeya should have been worshipped (or atleast widely known) considering the fact that two Gupta kings were named after him (Skanda Gupta and Kumara Gupta). Even today there is Karthikeya temple in Haryana at Pehowa. In Delhi, Malai Mandir is a temple dedicated to Karthikeya.
The existence of numerous temples lead us to the conclusion that Karthikeya worship is popular in South India. In Tamilnadu, it is equal in popularity to Shiva worship. But in North India, Karthikeya worship seems to have declined slowly.
Regarding the frames of Shiva's family, here is one with Karthikeya. In fact, this is the first picture that came up when I googled "siva family".