simply apply an offset equal to the total number of frames/2 depending on the direction they are turning/facing (use a variable to store the direction)
if direction=left then offset=0 else offset = (NumFrames/2)
*EDIT*
An excerpt from a piece of code I wrote a while ago:
if direction=left then offset=0 else offset = (NumFrames/2)
*EDIT*
An excerpt from a piece of code I wrote a while ago:
Select Player1\Facing Case 1 DrawImage Player1\Image, Player1\X, Player1\Y, Player1\Frame Case 0 DrawImage Player1\Image, Player1\X, Player1\Y, Player1\Frame+ ((Player1\nFrames/2)) End Select