When l'm opening this page https://multiverse.soulism.net/c/pixel_alchemists on the original server, it's showing up the images and the thumbnails, while when l open the same community from my feddit.online account, it's simply showing the titles.

Is it due to some difference in the CSS setting ??

I think you can change that with your settings :)

Here is the full personalization you can achieve with Piefed :
https://piefed.social/community/fediverse/wiki/piefed-user-css

Probably, you have compact mode on.

@Jerry@feddit.online would it be possible for you to uncompact it for my communities ?? They would look even more beautiful then.

Compact mode is a choice that is in your profile. It's not a server setting.

Go into Settings -> Compact UI and choose "Not Compact - Expand Images"

I checked. I've got my expand mode on.