Issue with File Permissions

I have been trying to make modifications to my plugins via WP and the FTP but I think there is an issue with file permissions.

I can’t edit, update, install and even delete plugins.

For instance, when I tried to delete a file in a plugin, I got the following message:
—
Response: 250 OK. Current directory is /wp-content/plugins/slider-image/images
Command: DELE lightbox-logo.jpg
Response: 550 Could not delete lightbox-logo.jpg: Permission denied
—

We’ve tried changing the ownership and the permissions but so far that’s not made any difference. Any help is much appreciated. Can show you more logs if it’s not clear what is going on.