What a coincidence! Last month I was scanning some posters from the late 1970s, which hung on my bedroom walls many moons ago. Extremely rare, mostly magazine pullouts. After scanning each corner and the middle areas of the posters with
my Epson V600 (a very high quality scanner for only $200!), and saving images directly to the desktop, I dragged the 6+ pieces into a separate folder on the desktop. Once that was done, I started Photoshop.
On this Mac, I have
Photoshop CS5. The full version, not Elements. In Photoshop CS5, you simply go to File > Automate > Photomerge. Select the images to be merged (go into your folder, hold SHIFT, and click the first and last one on the list to select them all), and the pick your merge method. Auto is often correct. The pieces will be warped, skewed, rotated, etc, as needed, to create the merge.
Photomerge has gotten better over time, too. CS3 was decent, CS4 improved more, but I can definitely see how CS5 is the best to date. I didn't merge photos very much, pre-CS3. (Prior to some CS3 projects, the last time I merged photos was before filters like this existed, in the mid 1990s.)
Now as far as un-yellowing goes, it may help if I had a sample of the scan. Generally speaking, in CS5 there's the nifty trio of filters under Image: Auto Tone, Auto Contrast, Auto Color. These are actually pretty decent. You can also alter levels and color saturation.
An alternative method (sometimes better, sometimes not) is to go into Adobe Bridge, and edit the JPEG with Adobe Camera Raw, and use the white balance tool.
