Can you please provide a little more information about what you are trying to achieve.

• What are the .png images and why do you wish to turn them off ?
• Is this only on a single site, if so what site ?
• Are you trying to follow some advice about this, if so where did that come from ? link ?

PNG is a format for (compressed) images just like the JPG format. There is no way to disable this format.

If you have a problem with images on some web pages then you may need to block images from some domains.

You can look at the Adblock Plus extension to block specific content.

• Adblock Plus: https://addons.mozilla.org/firefox/addon/adblock-plus/

You need to subscribe to a Filter list (e.g. the EasyList).

• http://adblockplus.org/en/subscriptions
• http://adblockplus.org/en/getting_started