How can display and change the parameters of an image (size, alt tags...)?

In order to display and alter the paramaters of an image or thumbnail, you'll need to use OpenTokens.

First, you need to put %OUI% as the very first characters of the input field. Then, you'd place the token.

For example, to display the product thumbnail with a set height and width you would use something like:

<img src="%VAR(Products.d.thumbnail)%" height="30" width="30" />

This option would replace the %TNAIL% token.