Path for images inside a block Right now I'm at "http://localhost:8080/drupal/xxxx/user/password" and I added an image inside a block like this "sites/default/files/background.jpg"

But I think is wrong because I'm at 2 paths "user/password" and it is not taken my images, in which way can I write the path to access to my images???
You can display image using following URL:

/drupal/xxxx/sites/default/files/background.jpg

This is for your local setup. While migrating your drupal setup to production at root directory, your image url will break, because of directory change. To avoid this, you will need to change the path of image as below:

/sites/default/files/background.jpg

You can fix this broken your without changing the URL at production by using Pathologic. After Installing this module, you will need to enable Pathologic filter by checking Correct URLs with Pathologic in Text Formats e.g Full HTML. Then select Correct URLs with Pathologic tab from vertical tabs &amp; Path relative to server root (/foo/bar) radio option. Add following values in the text area &amp; save.

/drupal/xxxx/
/