EDIT
I've re-read your question and realised that I misunderstood. A tool like Adblock might be the better option as it applies the blocks on a per domain basis, this would allow your new tabs to follow the same restrictions.
It is also easy to disable temporarily as required.
This used to be a standard option in the right-click menu until recent versions. Mozilla decided to move it to prevent accidental activation.
The steps to disable images now are as follows:
- Enter
about:config
in the URL bar.
- Accept the warning about invalidating your warranty.
- Enter
permissions.default.image
into the search box on the configuration page.
- Double click on the top option.
- Change the value from
1
to 2
to the pop up.
1
- loads images automatically (the default)
2
- disables loading images automatically
(source)
You can now restart your browser to allow the new settings to take effect.