OK. I understand your situation. THERE IS AN EASY SOLUTION TO MAKE IT FOR ALL THE 100 PIX within less than an hour, AND YOU DONT HAVE TO SIT AT YOUR MACHINE! You have to set only TWO photos. One landscape and on portrait.
1) Open Photshop CS
2) Open the 1st image in a normal way
3) Pick the "Actions" tab on the right side of the screen
4) Click the "New set" button below the "Actions" tab
5) Give the name: "My Actions"
6) Now click on the "Create new action" button
7) Give it the name "8x10 Portrait with no crop"
8) Image menu=>Image size. Set width and height to 75 *percent* (give by procentage it is better) Click OK.
9) Image menu=>Canvas size. Extend the height by 1 inch. Extend the width by 2 inch. Make sure the anchor is the center one. (In case of landscape image)
10) Now use the "save as" command (not the save) and save your example somewhere with a different name
11) Click on the "Stop recording" button below the actions panel.
Now comes the fun
12) "File menu=>Browse"
13) Find the directory where your files are located
14) Select all the portrait photos (hold the CTRL button and click on the image once)
15) Now go to the "Automate=>Batch"
In the pop-up window set the following:
Set: My Actions
Action: 8x10 Portrait with no crop
Source: File browser
Check the "Suppress File open dialogboxes", uncheck the others
Select a directory where you want to save your results (THE ORIGINALS WILL NOT BE OVERWRITTEN)
Check the Override Action "Save as commands"
And give a file name as you wish.
This is it you are done. Now if you click on the OK button it will do the same actions with all the files you selected AND it will save them to a specified folder with a new name.
Dont forget to do the same for the landscape photos. Use your intelligence. The whole procedure should last far less than one hour including the batch, when you dont have to sit at your machine.
And one thing, set the background color BEFORE you start the batch.