From the Safari menu bar click Safari / Preferences then select the Appearance tab.
Make sure: Display imagers when the pages opens is selected.
Actually it's a good idea to have another web browser installed on your Mac in case Safari isn't working. That way you can download and install updates or user that extra browser for troubleshooting.
As for the Tumblr extensions. When you install an extension on Safari.. they are accessible from Safari / Preferences - Extensions. You can turn Extension on or off as needed from the Extensions pane so having it installed might come in handy.