Custom default avatar not working

Hi, thank you for this plugin!

I am using a custom default avatar when users do not have gravatar nor uploaded one.

But… it does not work :/
Uploaded avatars are well displayed, but the default avatar is not. Instead of the avatar I have the text “image_alt” in a square.
here is the code it is showing in firefox:
<img alt="image_alt" src="#" class="avatar avatar-12 photo avatar-default" height="12" width="12">

What did I do wrong? 🙁
Thank you very much for your help! 🙂
Dorem

https://ww.wp.xz.cn/plugins/basic-user-avatars/