A Guru is literally a teacher or an expert. Sikh Gurus were a lot more than this in practice.
There were ten Sikh Gurus spanning the period 1469-1708.
Guru Nanak (1469-1539) was the first Sikh Guru and founder of Sikh religion.
Guru Nanak started the tradition of always having one Guru at a time to lead Sikhs during their formative years. Each Guru was successor of previous Guru. Not all of them were related.
Guru Gobind Singh was the tenth Sikh Guru. He ordered his followers to follow the Sikh holy Scripture Guru Granth Sahib as the eternal and final Guru.
No pictures or idols or representations of Sikh Gurus are allowed because Sikh religion opposes worshiping idols or glorifying personalities. Artist’s depictions of Gurus are frowned upon but generally tolerated if kept low key.
The current and final Sikh Guru is the Sikh holy Scripture Guru Granth Sahib. It has the status of an eternal and living Guru.