adobe photoshop - How to slice a huge image into smaller ones with 'Save for Web'

I have a huge image (1024*256000) which I want to slice horizontally into 100 images of 1024*256.

I'm using Adobe Photoshop CS6. I created the horizontal slices with the Slice tool and the Divide option.

Save for web allows me to save the images, but only at a 32.03% scale. If I try to change the Percent field to 100%, it gives me the following error:

A value between 0.01 and 32.03 is required. Closest value inserted.

Is there any way around this?