The question is ok to ask, the logic following is flawed however. How some human depicted/drew/painted Jesus as He prayed is not Biblical Truth, but a rendering of that particular person. Just saying. We can't base anything on the renderings of men to be Biblical Truth in my opinion.
That being said, as many have stated to bow and close one's eyes is a sign of reverence and respect for our Living King. For He surely is our King! One scripture given, there are others as well. The prayer of the Publican comes first to mind:
Luke 18:
10 .) Two men went up into the temple to pray; the one a Pharisee, and the other a publican.
11 .) The Pharisee stood and prayed thus with himself, God, I thank thee, that I am not as other men are, extortioners, unjust, adulterers, or even as this publican.
12 .) I fast twice in the week, I give tithes of all that I possess.
13 .)
And the publican, standing afar off, would not lift up so much as his eyes unto heaven, but smote upon his breast, saying, God be merciful to me a sinner.
14 .) I tell you, this man went down to his house justified rather than the other: for every one that exalteth himself shall be abased; and he that humbleth himself shall be exalted.
There are times I pray kneeling down, head bowed, eyes closed, and sometimes - even weeping. There are other times, when my arthritis is so bad that I can't kneel down, I will sit on the side of my bed with head bowed, eyes closed to pray. Other times, when asked to pray in Church Service, I will stand with eyes closed, head bowed to pray. Sometimes I spread open my arms, sometimes I don't. Other times I pray while driving, while walking around, or at the table asking Him to bless a meal, or many other times during the day. Much of the times such as these, my eyes are open.
Praying to God is NOT about a particular physical position, praying to God is ALL ABOUT the Spiritual condition of the one praying. We worship Him, and pray to Him............in Spirit and in Truth.
Don't look to the artistic renderings of man to learn how to worship God, look to the Holy Spirit.
God bless