Last update 1.8.7 break image support me
the bug is fix in the release 1.8.8
If you need to you can backport fix (use Image.LANCZOS instead of Image.ANTIALIAS for thumbnail resize by an0nfunc · Pull Request #15876 · matrix-org/synapse · GitHub)
by edit the file /opt/yunohost/matrix-synapse/lib/python
3.9/site-packages/synapse/media/thumbnailer.py
and modify the line like in the previous link.