The reason why a framed picture of Brahma baba (prajapita) combined with Shiv baba is kept at BK centres, is to explain that God (Shiv baba) who is incorporeal (shown as point of light), speaks through mouth of Brahma baba, who is the medium. Second, that knowledge is given, Raja Yog is taught and inheritance of new world is given by Godfather Shiv (creator of new world) through his 'rath' (chariot) who is Prajapita (ancestor, the first human being, Adam). Shiv baba (Baap) and Brahma (Dada) are combined and called as BapDada. This is shown in the photo.
You must have seen the above picture if you have visited a Brahma Kumari centre.
This and other similar pictures shows God, the supreme soul, as the light above head. Then it shows the corporeal or avyakt photo of Brahma just below Shiv baba. This explains that Shiv baba created Prajapita (first human soul/being) and named it Brahma. And using the medium of Brahma, then Shiva creates the new world (Golden age/heaven). Thus the heaven is known as 'Shivalay' in Hindi.
The entrance of Shiv baba (divine light) in the mastak (head) of corporeal Brahma is depicted in below image.
Now, there are many new BKs (Brahma Kumari and Kumar) who kept this trans light photo of BapDada in their home and workplace. This is however, not important and not said in Murlis. Shiv baba has instead warned - NOT to keep any human beings photo anywhere. Not in your house, not in your room, etc. This all is body consciousness.
We are studying the godly knowledge from God himself. God, being incorporeal, needs a medium in whom he can enter and speak. According to the World Drama created, the medium is fixed - it is the first human soul who is remembered till date as names like Adam (in Bible and Quran) and Aadi Dev in Hindu scriptures.
Prajapita Brahma is also an ordinary human being, a creation, a student of the incorporeal Supreme Soul (Shiv). We call baba to Shiv baba. The word 'baba' means 'Father'. He is the father of all the human souls. His name is eternally 'Shiv', which means 'Benevolent' or 'Doer of good to all' (Kalyaankari).
What does the Picture explain?
God, being incorporeal (one without a body) needs to medium to speak to us children. Brahma is the chosen medium (chariot) through whom he gives the knowledge of entire creation and the creator and liberates all the souls from sorrow. Through Brahma, he gives the imperishable knowledge and inheritance of the golden age (heaven), which is the first part of world drama cycle.
Please know that photo of Prajapita Brahma is kept at centres ONLY TO EXPLAIN that Shiv baba (supreme father of all souls) has come and speaks the Murli (knowledge of truth) through this medium - 'Prajapita' (ancestor of all human beings, Adam)
Even photo of Shiv baba (incorporeal trans-light image) is not required. All this is body consciousness. Shiv baba is giving the spiritual knowledge to make us Soul Conscious. This knowledge is an awakening towards the true self identity and the story of world.
-- In the light of Gyan Murli ---
''You children are becoming soul conscious and your aim is to bring the new world so that all souls of world gets back to their home and find peace. Then you shall come in your kingdom of happiness. You must do this service of giving the message and in this, you must forget body consciousness. You should not see this corporeal body, nor keep a picture of any human being with you, not even of this Brahma.''
~ Sakar Murli (Shiv baba original version)
If you need a photo (HD translight image of Shiv baba) for any other purpose, for example - to do service, then you can get it here, and then print it: https://www.brahma-kumaris.com/shiv-baba-wallpaper
---- Useful links ----
What is Gyan Murli?: https://www.brahma-kumaris.com/what-is-murli
Introduction of Shiv Baba, our Spiritual Father: https://www.brahma-kumaris.com/god-the-supreme-soul-shiv-baba
Biography of Great Souls: https://www.brahma-kumaris.com/biography
Hindi - 7days RajYog course: https://www.brahma-kumaris.com/raja-yoga-course-hindi