Add configurable image sizes

[erral]

We need an option to select the size of the image:

• Get the available sizes from the backend to a dropdown
• Use the selected size in the view

[ionlizarazu]

@erral I would not add this function in the view. I think is better to add it in the config file, it's not a end-user critical setting. @libargutxi ?

[erral]

But if we set it in the config we can't use different image sizes in different listings. We need to have it configurable per block instance.