Use the de-interlace filter in photoshop. 

 If you want the best possible image then you don't want to merge the fields.  Export a still with both fields.  If there is any movement between the two fields then selectively remove the movement in e.g. photoshop

regards

Brian

bsuttie is correct.  If there is little or no movement in the frame don't de-interlace.  Try all the options using the de-interlace filter and go with what looks best which may very well be nothing.  The other thing to consider is that if you send out a 720x480 image it will have to be resized to correct the aspect ratio if it's being used for non-video purposes.  You can make it 640x480 or stetch it to 720x540 in photoshop.