How can I retrieve category thumbnails to use in my module?

[trouille2]

Hello everybody, 

I'm pretty new to prestashop and try to create a module that would show a fullscreen background picture. Background should change following the category I visit and even choose randomly between 3 images for each category.

I plan to create an array containing the category thumbnails url originally set up in blockCategories module (I know it's not the purpose of these thumbnails, but I don't use them anyway in this website). 

 

For now, I've created a new module that can show a static fullscreen background (1 image place in img folder),

Next steps are (I think):

1. Get the category id of the page I visit or (if I'm in a product page), the category id I come from.
2. retrieve the category thumbnails,
3. create the array.

Who can help me for the 2 first steps?