variable alt attribute for thumbnail

[TobitRE]

Hi,

how can I use a variable inside the thumbnail template tag?
I am following the tutorial:

{% load imagekit %}
{% generateimage 'myapp:thumbnail' source=source_file -- alt="A picture of Me" id="mypicture" %}

this works fine but now I want to fill the alt attribute with a variable.

I try it with a {% with %} condition and without. Nothing works. The image tag include the variable name without replacing it by the value.

thanks for helping.

best regards