Why do Orthodox Christians baptize infants? I know that Roman Catholics baptize infants, because they believe that infants need to be cleansed of original sin, and that the denial of baptism at infancy by Christian parents is tantamount to the denial of God's grace for the child. So, why do Orthodox Christians baptize infants then? And how come they practice baptism by immersion on infants, as shown in this photograph? 

These are related questions.
Because it is Tradition.  Exactly why the Tradition started, I don't know, but here is a present-day explanation for why to do it:

In the Orthodox Church when a child is baptized they are also Chrismated, which I guess is similar to a Catholic Christening.  It is for sealing them to protect against evil spirits and such.  The Baptism / Chrismation is the 2-part process of cleansing a child of anything evil and/or sinful and then sealing it to keep in the good and keep out the bad.  It's like, you wouldn't seal something off before decontaminating it, right?

So then that usually begs the question about whether a child that young can actually be "contaminated".  Well, consider this: if an evil spirit can influence or possess someone, what makes you think they could not do so to an infant?  Who can know if even by the time an infant is baptized, something bad hasn't tried to attach itself already.  And why would you not want to do all that you can to protect your child from evil?

I've heard people make the argument that a child shouldn't be baptized until they are old enough to repent and understand repentance, because in Acts 2:38 Peter says "Repent and be baptized...".  But in context, he is addressing a group of adults - who at that age should of course repent first...that does not necessarily mean that their children were not also baptized.  In fact the very next verse says the promise of the Holy Spirit is for them and their children.

Note that the Orthodox Church also practices Confession, and children don't give their first confession until a certain age.  So if you are at least that old and have not been baptized, you confess first...if you are too young you get baptized first.