There are great editing tools online, so you don't need to have Photoshop to do any of this. If you're not used to editing, the first thing I would advise you to do is play around with your images till you know what look you want, and what works or not. Put them side by side, on top of each other, cut them... Imagine you are scrapbooking. Remember, what we picture in our head doesn't always work visually, so it's worth playing a bit.
I find that an easy way to do this is use Powerpoint. Office aplications are something everyone is used to deal with, and as access to, but why Powerpoint? Well, this is an application that allows you to make compositions on screen, so you can easily move objects, resize them, cut them... I could as well recommend Publisher, but for what matters you can do just the same with Powerpoint, with the advantage of using a much more comfortable tool that you're used to. Once you've mastered it you can skip this and use photo editing tools, but, whenever possible, play with the composition first. It is very useful for you to visualize the end result quickly, and try different things, where photo editing tools will take you longer.
Tutorial 1 - for beginners
Diptychs with MS Powerpoint
1. Open Powerpoint and create a new blank presentation.
2. Insert your pictures on a blank slide and start playing with them.
5. Make sure your images are perfectly aligned and that the sizes are matching (if you cropped them). Use the align button on the 'Format' tab while having both images selected to align them.
I suppose you can do just the same with OpenOffice, but I never used that, so I cannot tell. Feel free to ask any question if you need any kind of help. I'll b be back tomorrow with another Tutorial for diptychs with Pixlr and Photoshop.
all photos by me