How to line up images on both sides of the canvas to create and print a double-sided post card

Wouldn't making a template work and just laying each image on that? That's what I do (illustrations including greeting cards with two sides).

-edit- Must be different for photos or something?

It all comes down to how repeatable and consistent the second print is compared to the first print. If consistent you know in which directions and how much to fudge the placement of the second page elements.

Front to back image alignment comes with a price, often more expensive hardware is more consistent.

Are we understanding the problem correctly? Like are you not just talking about crop mark alignment between front and back, but also for example an image at the top left corner of the front being aligned with an image of the same size at the top right corner of the back when you turn over the sheet? In that example, it should just be a matter of reflecting the layout coordinates for front and back. As long as the documents for both sides use layout measurements that are a mirror of each other, they are exactly aligned on both sides at the document level, because the document stores the coordinates precisely. Any misalignment is caused by the paper feed hardware in the specific printer you use (see below).

One possible way to simplify that: In the template for the back document, you could drag the ruler zero point to the opposite edge so that, if in the front document you have an layer 1 inch from the left edge, on the back document you simply position a layer -1 inch from the right edge of that document (since the zero point was moved there), and they are aligned front and back.

What kind of printer are you using? For a lot of home/home office printers, there’s no built-in way to guarantee that each sheet is going to feed in exactly the same way as the other sheets. You’d have to use a printer that is engineered to guarantee precise registration every time, like a printing press can.

If the automatic paper feed on your printer cannot precisely feed each sheet consistently, sometimes it helps to use manual feed instead. For example on my printer, before doing a large print on expensive paper, I will manual feed a sheet, open the printer lid, look at where the leading edge is to make sure it’s aligned exactly where it should be for the start, and if it isn’t, feed it again.

I’ve done double-sided postcards and greeting cards on my Epson photo inkjet printer. The double-sided jobs I’ve done have been “close enough.” My printer is one of the many that can’t guarantee registration from sheet to sheet, so I try to design so that it’s OK if the printer is off by a millimeter or so on the other side. 

I have found that the only way to achieve this is by testing prints - and keeping expensive accurate notes. 

That way you'll come up with a way that works.

I hope this helps
neil barstow, colourmanagement net :: adobe forum volunteer:: co-author: 'getting colour right'
google me "neil barstow colourmanagement" for lots of free articles on colour management

Think about using a program that is designed for multi-page printing such as InDesign. Its much easier to align signatures when there are built-in features to do so.

Select both layer in the Layers panel and use the alignment buttons that will appear in the options bar when you have multiple layers selected.

If your issue is different, show a screenshot with your image, Layers panel, and options bar so we can better assist.

Jane