Many times in the life of a graphic designer it is necessary to scan a large image and then piece it back together in Photoshop.
Begin by making multiple overlapping scans of your image; try to make them along a horizontal and vertical axis. Now if your image has a round or organic edge, adhere it to a squared edge piece of board for scanning purposes. Depending on the size of your image you may have any number of scans.
Take a minute to look at all your scans, look to see if any of them are not at 90 degree angles. Select the ruler tool.
Now click and drag a line with the ruler tool along an edge that is supposed to be horizontal.
Go through Image->rotate canvas-> arbitrary->.
The rotate arbitrary dialog box opens up and begins with the angle created by the ruler tool.
Hit OK. Now your scan is ready for combining with others. Repeat this process for all of your scans.
Now you need to get all of your images facing the same direction, so you may have to rotate them 90cw (clock wise), 90ccw (counter clock wise), or 180 degrees. Take a scan that needs to be rotated.
Go to Image Rotate Canvas and pick the corrosponding degree and direction.
Now your image is facing the correct way.
Repeat this process until all of your scans are facing the same direction.
You are now ready to combine them all! Start with a scan that is on one far corner of your image, if your image is round start on any edge. Increase the canvas size to a little bit larger than your final artwork through Image->Canvas Size.
The Canvas Size dialog box opens up.
Here you can put in your new size, the area on the bottom lets you control from which sides the canvas will grow. If you choose center your scan will be centered in the page with excess area on every side. I am choosing the lower right edge as my current scan is also on the lower right edge of the artwork
Take a scan that is adjacent to your first scan and drag it into your original scan. This will make a new layer in your first scan image.
Adjust the opacity of the second scan layer to roughly 50% (this does not have to be exact, so don’t spend too much time trying to make it so, just somewhere close so you can see both the original scan and the new scan layer clearly).
Zoom in close and use the move tool 
to move the second scan around (make sure the second scan layer is selected). Use the mouse/pen to get the second scan close to lining up with the first.
Then use the arrow keys to make minute adjustments until the image becomes one clear solid image.
Zoom back out and increase the opacity of the second scan layer to %100.
Pick the paint brush tool, from the bar at the top choose a soft edge brush and from the mode drop down pick clear, and opacity of 15%.
Use this paint brush on the second layer to clear off the hard edge of the layer where it overlaps the first layer.
When done if one scan is darker than the other or slightly different color, use Image->Adjustments->Levels to make it match the first image. You may also need other Image Adjustments tools to make slight adjustments to make it match the first scan.
Now repeat this process with all of your scans. Then flatten the image by Layer->Flatten Image or using the drop down menu from the layers palette.
Use the crop tool 
to crop to your final combined image. And finally you are done; here is a screen shot of my image that is 22” x 30” scanned with a scanner that has an 8.5” x 11” scan bed. This particular piece is called Pig Baby and is part of a series of drawings and hopefully paintings inspired by Alice in Wonderland.
