I suggest that you make all the icons and buttons the same size, so page turning button, main category button, the subcategories and the thumbnails.

I liked the old subcategory pictures, it was easy to choose with the icons.
Can you do two versions of the subcategory icons, one with only picture and one with picture and title? To see which one is better.

The bigger subcategory icons would push the thumbnails lower, because I think the bottom of the thumbnail field should align with the bottom of the generated image.

Also if you have the navigation icons on top, in big, you don’t need the small arrows on the bottom. The page numbering is nice there, but no small triangles.