How to use a new image size in default theme?

[chipset]

Hello,

i wish to change something in my images in the home page, because Prestashop 1.6.x resized it to other size. I use default theme.

The problem is that Prestashop use home_default (250x250, resized to 175x175) to home page and category pictures (250x250). If i change it to 175x175, the category images are small.

I can add new image size 175x175, but i don't know where to change in Prestashop core to use them in the home page.