Im not really sure exactly if your talking about, mirroring the geometry, or aligning the image in the background. So I’m going to cover the geometry aspect, and if I don’t cover what you need then my apologies.
you don’t necessarily need to use an instance to create the other half of your geometry. But, regardless, I’ll explain this, because instances can be useful. First of all to create your instance, Let’s assume your model is facing the toward the Z axis so that when you click front, you will see a perfect view of the front of your model. If it is not, then go to the attribute editor and in the transformation attributes section make sure that all three of the Rotate boxes (x y and z) have 0 for a value.
Okay, the next question to consider deals with the side you want mirrored, remember, we havent yet created an instance so this should be your only side. Are the border edges of your model perfectly aligned with the y axis. All of your border edges should be connected in a perfectly straight line so that everything is even, if not, no matter what method of mirroring you use, your going to have some serious problems. When we get to creating the instance, this may not actually matter, but when working with maya, its important to keep everything neat. If not then look at the top of the screen at the main toolbar, you’ll notice about four or five symbols with magnets. This first one which is a magnet overlaying a grid (not the third one which is a magnet hovering over a skewed grid) is the snap to point tool, with it you can snap verticies and edges literally to any of the squares on the Maya’s grid. Just make sure that the model is clean.
Now lets make the instance. Select your model, and goto edit and click on the “Duplicate special” options box (an options box, is the little square next to the majority of menu items in Maya (If you already knew this as well any of the other explanations and are rolling your eyes, please don’t take this as me talking down to you, I just dont know your current skill level and am just trying to break everything down as simply as I can.)) Okay so we’re in the duplicate special options menu, under “Geometry type”, your going to check Instance, and under “Group Type”, click world you can click either parent or world, I recommend world, it’ll keep the geometry seperate, and parent gives me trouble from time to time.
In the transforms sections, you’ll notice three boxes each for Transform, rotate and scale. These three boxes are the x y and z specications for what your making. Translate is move, Rotate, is obviously rotation, and scale is the size. Set all three values for both Translate and rotate to 0, since you don’t want it to move anywhere new, or facing a different direction. For scale, set x to -1, y to 1, and z to 1. This will ideally produce your new instance in the correct orientation and position. Now I ran into a slight hitch while I was testing it out in my character, If your noticing that your instance is not appearing directly opposite the right side, or is physically within the right side, your pivot may be off. To see what I mean, select your right side, and then select the translate tool. You’ll notice that its not quite in the center, if you press home on your keyboard, you’ll notice that the arrows on the translate key will dissapear, and a small circle will appear in the center. Now if you notice, when you move the translation tool your model does not move with it. Bring this to the center, ideally: goto front view, and click the snap to grid tool again and move the tool so it snaps to the center of the grid along the y axis. now, lets create the instance again, unless of course you didnt have this problem, but if you did, be sure to delete the original instance you made last time. It should work.
Combining the two sides.
Give a little bit of space, between both sides of the model, just enough so you can see a little grey between both sides. shift select both sides, and click “Mesh” and then “combine” to make them one complete model. Now we still have a space in the middle, theres two ways to fix this, you can either select a border point on the right side and then shift select that same point on the left, and click “edit mesh” and then “merge”, and do this with every pair of verticies on your model, which is a sure fire way to success but is very long and tedious, or you can goto “edit mesh” merge border edge tool, which is much quicker, but a little more difficult.
The Proper Solution
If you goto the Mesh tab (I’m using maya 8.5, so if your using an earlier version, the tab that I’m talking about is polygons, though its been a while since I’ve used an older version and I could be wrong.) click mirror geometry, which will bring up a menu. In this menu you can specify which direction your mesh will be mirrored. If your model is facing foward on the z axis (the default) then -x will produce a full version of the model (this is ofcourse, if you deleted the left side of the model, if you deleted the right side then +X will do the trick.)
If you then look at that menu there is an option that should already be checked called “Merge with original”. What this means, is that when the other half of the model is created, it will automatically be joined with the original half into a full model, meaning ofcourse that both sides are perfectly attached. The other two options, under Merge with original, specify whether the attaching is done by connecting the verticies, or the edges. Sometimes you’ll get mixed results, so be sure to experiment with either, and play around with the settings to see what you’ll get.
Hope this helps.