Oh thats easy if you know...
BitBlt. That and the other windows API graphics functions are your answer. Although if your making something as big as Red Alert you may wanna learn DirectX.
Once you know BitBlt's brother StretchBlt:
Private Declare Function StretchBlt Lib "gdi32" Alias "StretchBlt" (ByVal hdc As Long, ByVal x As Long, ByVal y As Long, ByVal nWidth As Long, ByVal nHeight As Long, ByVal hSrcDC As Long, ByVal xSrc As Long, ByVal ySrc As Long, ByVal nSrcWidth As Long, ByVal nSrcHeight As Long, ByVal dwRop As Long) As Long
Err, ok, I'm not gonna explain all that, but as I was saying, once you know stretchblt you can see that you specify the Start X and the Start Y and then the width and height within the DC (picturebox) that you are copying from. I.e. you specify a selection within the picture that you are copying from and then you copy it to another DC (picturebox)... or even to a different position in the same DC.
Damn I'm bad at explaining things. Hope you understand.