Looking for a good image resizing program

I recently put up an e-commerce site and as part of entering products I can input images in 3 sizes.
1. 100 x 80 which is used for photos put on the main page of the website to be clicked on for further information

2. 200 x 250 to be used for the image in the actual product description

3. 300 x 400 to be used as a larger photo as a pop-up picture if someone wants to see a larger picture.

The pictures can vary in size but unless the variation is small the pictures will all very in size on the main page of my website which will give it a bad appearance.

Unfortunately when I get stock photos many vary in size. I want to know if there's an easy way to resize pictures. The only concerns I have is that making them larger may make them appear stretched or making them smaller may give the opposite appearance. Also, since I am doing this to many pictures I want a solution that is quick and easy, maybe as simple as taking a picture, choosing a new size, and saving it with the change.

I currently have the Adobe Creative Suite 3 with PhotoShop, Illustrator, and Fireworks but am not sure if this does it in an easy way and of course I prefer to use Mac over Windows. Any suggestions of a good and easy way to do this?

 

I'll look further into Automater and I also heard that Preview can resize.