Resized high resolution product images, now have jagged edges

I'm ready to start inputting my product images into my build, I have high resolution images, large of course, 2550 x 2550. I "thought" all I would have to do is to batch resize them in Irfanview. I have 250 images, and want to be able to do it in one process.

I resized the images to 600px, set it to the highest quality in .jpg format and resized.

I'm looking at a vest I resized. It's a blue vest on a white background, not only are the edges jagged, the color in the body of the vest looks wavy and distorted. I'm enclosing a picture of it.

Can someone give me some help. Should I use another program to resize these images? Or is there a plug-in in Irfanview that would help me correct the problem?

I'm confused, once again, when I thought I was making great strides forward. Thanks in advance for any advise you can offer me. You have all been great at helping me with this.

Update:

I went into Irfanview, resized this vest individually to 600px, using both .jpg and .png formats, viewed it and it doesn't distort. It renders properly.

Apparently, it's just when i batch resize that I have problems, but I don't want to do all of these individually if I don't have to, there are a lot of pictures.

Advise, please..

ok
Image handler2 will create 3 images based on the sizes you desire to display in the cart
For a thumbnail, for the product page and for the listing pages
here is the link to image handler 2
http://www.zen-cart.com/index.php?main_page=product_contrib_info&products_id=117

You can resize it you self and rename it image_MED.jpg or image_LRG.jpg

Any large image that is down sized by html will be re-sampled in your browser and it can get ugly... (Firefox is the worst in this aspect)

The problem was I didn't check the Resample function (better quality) in the advanced options of Irfanview. Maybe this will help somebody else out. When I checkd it, conversion is smooth.