Photoshop works well, but Paintshop Pro works as well.
I just photoshopped a 47 photo aerial imagery run into one big image (they all overlap), so I think I know what you're trying to do.
This will be pretty meaningless until you get your head around using the software, but here goes...
1. create a new blank or transparent image and specify the size e.g. 12" x 12". Set the resolution to around 300 dpi
2. Open your middle image in photoshop (leaving the new blank image open).
3. Hold down the ctrl button on your keyboard and press the A key once - that puts a rectangle around the whole image - known as "selecting" it.
4. Hold down the ctrl button and press the C key once - that copies everything inside the rectangle.
5. Go to the window menu and click on the blank image (should be called untitled.psd) - this brings it to the front.
6. Hold down the ctrl button on the keyboard and press the V key once - this pastes the copied image into blank one. It should end up in the very middle of the blank image. If it's really small, then select the magnifying glass from the toolbar (zoom tool) and click on the image until it fills about half the screen.
7. Go to the file menu and click "open". Find the left side image and repeat items 1 through 6.
8. On the toolbar you'll see an icon which looks like a black cross with arrow ends. Click on this - it allows you to move the left side image around the screen. You can now place it in as close to the right place as you can get it.
9. Go to the file menu again and click open. Find the right side image and repeat item 8.
10. Hopefully now you've got your pictures stitched together. They might be a little bit out of whack no matter what you do (aerial photos are like this), and there are ways to fix it beyond the scope of my instructions here.
If you look at the "Layers" palette (bottom right of the photoshop screen), you'll see that there are four layers. Unless you want to fiddle around some more, you're done and you need to flatten the layers. To do this, go to the "Layers" menu and select "Flatten image". Check the layers pallette again and you'll notice you only have one layer.
To save the picture you have two main options -
1. Save as a BIG jpeg file - go file > save and choose Jpeg
2. Save for web - go file >save for web and choose jpeg again - what this does is reduce the file size so you can email it and such.
Of course, photoshop has a pretty steep learning curve, so maybe just download and use Autostitch -
http://www.photo-freeware.net/autostitch.php
Not as accurate, but it should work OK.
Hope it helps.
