This information is obtained from various online sources and scholars who have done sufficient research in this area.
Rigveda (Mandala X, Sukta 130, Mantra 3), we have the statement
'kAsIt pramA pratimA'
which means 'what was the size and what was the deity form (mUrti form) of that God?'.
Sayana's Bhashya for 'pratimA' here goes as 'pratimA means the vigraha form which the God takes in order to accept the sacrificial offerings in a yajna' (haviH prati yogitvena mIyate nirmIyata iti pratimA devatA'.
In addition, there is a description of the yajna called 'iShTishayanaM'. (Taittiriya Samhita 5.2.7) There is an instruction to conduct the yajna with a form similar to that of Garuda using bricks. Here, we also find the following statement:
'If one worships the golden mUrti of Purusha seated on the mUrti of that bird, the conductor of the sacrifices (yajamAna) obtains all glory'.
This is an obvious reference to Lord Vishnu seated on His mount, Garuda."
In addition, it is said in the Ramayana that Rama worshiped Lord Vishnu (Himself) in the mUrti form inside ayodhya (in Ayodhya Kanda).
In this worship, Valmiki states that Lord Rama performed a Vedic yajna in the temple for the purpose of this worship. In addition, when Rama goes to the hermitage of Agastya (in Aranya Kanda), he finds that Agastya, the sage, had consecrated the mUrti forms of many Vedic devatas.
These two incidents also support the fact that mUrti pUja is supported by the Vedas.
Also, in Vyasa Mahabharata has references to deity worship of Siva by Aswatthama.
So, Deity worship is as as old as Vedas themselves.
It is truly unfortunate, that well known individuals and modern day Gurus make irresponsible statements' like there were no Idol or deity worship in ancient Indian scriptures (Here I am assuming what Marcus has quoted is true)
Even, Linga worship is nothing but deity, saguna worship only, however hard and deseperately one tries to argue Nirguna and what not.
Such irresponsible statements will only strengthen the Abrahamic religions and in the process might hurt our Sanatana Hindu dharma, in the long run.
Now coming to Raama worshiping Siva.
As per Valmiki Ramayana 6.123.19 verse of Yuddha Khanda is as follows
etat kukShau samudrasya skandhaavaaraniveshanam || 6-123-19
atra puurvaM mahaadevaH prasaadamakarotprabhuH |
Now many people jump to conclusions that this word Mahadeva word refers to Siva only trying to infer meanings by referring to contexts where Siva is addressed as Mahadeva.
This Mahadeva word can also be an epithet used to address Lord of Ocean in this context, as there are many instances in Vedas, ithihasas, puranas where common names like Mahadeva, Iswara, Maheswara etc are used to address other deities also.
Commentaries on Valmiki Ramayana by Maheswara tirtha (Advatin) called tattva dipa and Bhushana of Govinda Raja clearly say that the "Mahadeva" in the verse 6.123.19 of Valmiki Ramayana refers to Lord of Ocean, Samudraraja.
Maheshvara Tirtha, the advaitic commentator, says
mahAdeva is samudrarAja only.
He also says, even if mahAdeva is Siva, it only refers to his aShTamUrti form as water, who served bhagavAn by bearing the rocks that formed the sEthu.
maheshvara tIrtha further shows why mahAdeva is samudra?
(1) It is more suited to the context,
(2) according to nighaNTu shAstras, "mahat" is a name for water, and hence "mahAdeva" is the deity who controls the water.
With all the above proofs, it is safe to conclude that Mahadeva in verse 6.123.19 of Valmiki Ramayana refers to Lord of Ocean or Samudraraja only.
To further add to these the first Ramayana in vernacular, Indian languages, Kamba Ramayana by famous Poet Kamban in tamil in 9-10 century doesnt say anything about Raama worshipping Linga or Siva.
From all these it is very clear there was no worship of Siva or Linga by Raama in Valmiki Ramayana in Yuddha Khanda.