I would agree with PD. I think it was done to try to normalise the act of Muslim prayer to a mainly non-Muslim population as the actions of prayer seem really normal to Muslims but looks a bit strange if you are not familiar with it. Possibly Yousaf thought this picture may go some way to making it seem less unfamiliar, strange, worrying to see men lined up to pray. I remember a while ago reading stories about a few people freaking out on planes in relation to Muslims engaging in normal religious acts e.g reciting the Quran in Arabic on a plane.
Indeed - and there is a short piece on the BBC live stream linking to the tweet which is basically an education piece for people unfamiliar with islamic practice:
"Humza Yousaf's family are among millions of Muslims in the UK and around the world who are marking Ramadan, one of the holiest months in the Islamic calendar.
It involves a period of fasting before Eid al-Fitr, which translates as "the festival of the breaking of the fast", on 21 April.
Muslims have an early morning meal before dawn, known as suhoor or sehri.
They do not eat or drink anything - including water - until they break their fast after sunset for the evening meal, called iftar or fitoor."
I suspect there are plenty of people who have little understanding of this, but it is helpful to have this brought out into the open more - not least because many workplaces will now have muslims who are fasting and small adjustments to workplace practice (including simply being sensitive) can go a long way to support an inclusive workplace that functions better.
It is also possibly a way of relating to the Muslim population by mentioning fasting and showing them lining up to pray i.e. to say to the Muslims that Yousaf's faith is still very important to him and he is still relatable - he is doing what a lot of them would be doing in their own houses. Possibly because many Muslims will think there are policies he supports that would seem at odds with his faith - according to their interpretation.
Perhaps, although I'd be rather less convinced that the muslim community in Scotland needs much educating on the importance of Yousaf's faith - I think he's been pretty clear on this throughout his time in politics, not least wearing traditional dress for some ceremonial situations when many would have just worn a suit.