This indicates the Brahmacharya of Hanuman ji.
In Hinduism,the left side/ left lap of a male married deity/a father deity is reserved for his spouse(ardhangini)
Two instances where this concept is highlighted:
a)In the Vishwanathashtakam,Adi Shankaracharya describes Lord Shiva as “Gauri Niranthara Vibhushita Vama Bhagam”(He who is adorned by Gauri on His left perpetually)
b)In the Rama Raksha Stotra,31st verse is as follows:
“दक्षिणे लक्ष्मणो यस्य वामे तु जनकात्मजा...”
But Hanuman ji is a brahmachari,hence His left lap is not to be filled as such,hence it is depicted in Such a way that sitting on it is not possible ,in the position that it’s shown.
(Hanuman ji does have a spouse who is the daughter of Surya,but since He did not father children,this may be the reason she doesn’t sit on His left lap)
You can find the same logic in depictions of Lord Ayyapa.
He is also a Brahmachari,so His left lap is positioned in such a way that in the position that it is,it cannot be sat upon(signifying His celibacy/Brahmacharya)
Lord Ayyapa is a “naishthik “ Brahmachari,hence His right lap is also in that position as is His left lap(Hanuman ji’s laps are placed differently,yet the left lap is placed in both Their icons,in such a way that in the position that it is shown,it cannot be sat upon.
I doubt as to whether the kneeling posture shown in the picture (or for that matter,any single posture) is the proper mandate for depicting Hanumanji,as I’ve seen pictures where the knees are placed reverse with respect to what the question shows:
Left knee folded:
This is loosely connected to the guideline of the left foot at the mulabandha(as shown in the comments section)
Right knee folded:
Both knees folded:
Hanumanji in Padmasan:
Given that Hanumanji is depicted in all these ways,I don’t think that all the four postures shown have their own special meaning,**when Hanumanji is shown in that posture,in a way so as to depict some quality of Hanumanji specifically.
(Last two photos shown are Vajrasan and Padmasan respectively,but again I don’t think there is any specific trait of Hanumanji that they show especially )
So in conclusion,from what I could gather until now,there is no set way to depict Hanumanji,and this is the artist’s choice..
(Please correct if I’m wrong)