After activating all necessary modules you need to assign them to correct positions. In PrestaShop admin panel from the top menu select Modules > Positions. To place module in the required position just drag it to the position section of the modules list.
In PrestaShop you can use predefined image dimensions for product images, category images, manufacturers etc.
For example, images of the 'home' dimensions are usually used for the category listing product images, 'large' for main product image on the product info page.
To configure product image dimensions, open your PrestaShop admin panel, go to Preferences > Images and set the image dimensions as displayed below: