Try to use sorl thumbnail, it even supports Amazon S3 Server.
pip install sorl-thumbnail
To Installed apps: 'sorl.thumbnail',
add {% load thumbnail %} to your template.
{% thumbnail firma.firma_logo "130x110" format="PNG" as im %}
<div class="logo" style="background-image:url('{{ im.url }}')"></div>
{% endthumbnail %}
The make image format PNG is important, it converts into JPEG by the default. And if the user uploads file in transparent format, it sucks.