Basically, Harimandir Sahib is holy because it's a place of singing praise of God:
ਗਉੜੀ ਮਹਲਾ ੫ ॥
Gauri 5th Guru.
ਧੰਨੁ ਇਹੁ ਥਾਨੁ ਗੋਵਿੰਦ ਗੁਣ ਗਾਏ ॥
Blessed is this place, where they sing praises of the World Master.
ਕੁਸਲ ਖੇਮ ਪ੍ਰਭਿ ਆਪਿ ਬਸਾਏ ॥੧॥ ਰਹਾਉ ॥
The Lord Himself Blesseth them with peace and pleasure.
But a photo isn't that. It's not the real thing. Similarly, a statue of a deity is not the deity. So we shouldn't be worshipping the photo.
One type of partakh (immediate) darshan of Guru ji is going to the Gurdwara to see Guru Granth Sahib ji. Because the Book is the abode of God:
ਪੋਥੀ ਪਰਮੇਸਰ ਕਾ ਥਾਨੁ ॥
This Holy Book (the Adi Granth) is the abode of the Transcendent Lord.
ਸਾਧਸੰਗਿ ਗਾਵਹਿ ਗੁਣ ਗੋਬਿੰਦ ਪੂਰਨ ਬ੍ਰਹਮ ਗਿਆਨੁ ॥੧॥ ਰਹਾਉ ॥
Whosoever sings the World-Lord's praise in the society of the saints, he is blessed with the perfect Divine knowledge. Pause. p1226
There is another level of Darshan which Guru ji pines for and which is above merely going to the Gurdwara:
ਕਰਿ ਦਇਆ ਮਇਆ ਦਇਆਲ ਸੁਆਮੀ ਹੋਹੁ ਦੀਨ ਕ੍ਰਿਪਾਰਾ ॥
Show compassion and kindness, O Merciful Lord, and by king to the meek.
ਬਿਨਵੰਤਿ ਨਾਨਕ ਦਰਸ ਪਿਆਸੇ ਮਿਲਿ ਦਰਸਨ ਸੁਖੁ ਸਾਰਾ ॥੧॥
Supplicates Nanak, I am thirsting for a sight of Thee. By obtaining a view of Thine I receive all the happiness.
This Darshan is found through Bani and worship:
ਅੰਮ੍ਰਿਤ ਬਾਣੀ ਹਰਿ ਹਰਿ ਤੇਰੀ ॥
My Lord God! Thy Gurubani is Nectar.
ਸੁਣਿ ਸੁਣਿ ਹੋਵੈ ਪਰਮ ਗਤਿ ਮੇਰੀ ॥
By continuously hearing it, I have obtained the highest rank.
ਜਲਨਿ ਬੁਝੀ ਸੀਤਲੁ ਹੋਇ ਮਨੂਆ ਸਤਿਗੁਰ ਕਾ ਦਰਸਨੁ ਪਾਏ ਜੀਉ ॥੧॥
The burning within me is extinguished and my soul is rendered cool on having the sight of the True Guru