@maeva: If you want to have it in the footer widget area, this can be down with Text Widget.
1. Go to Dashboard=> Media=> Add Media and upload the logo you want to display in the footer.
2. Once the upload complete you’ll see edit button, click it.
3. A new tab will open and you’ll see file url and other option in that page at the right side. Copy the file URL.
5. Go to Dashboard=> Appearance=> Widget then add a Text Widget to one of the footer area and then copy and paste the following code into the widget.
<div class="custom-footer-logo">
<a title="custom-footer-logo" href="yourlink.com">
<img src="paste-copied-file-url-here" alt="custom-footer-logo">
</a>
</div>
Note: In the above code, replace yourlink.com and paste-copied-file-url-here with your desired LINK and the url you’ve copied earlier after uploading the logo in the media respectively.
Let me know if any trouble.
Regards,
Mahesh