To disable images on Yahoo mail:
Here’s how to block images:
1. Click Options in the upper-right corner of your Mail page.
2. Select Mail Options.
3. From the list on the left, click Spam.
4. In the "SpamGuard" section, find "Block images:".
5. Select Initially block all images. You’ll have the opportunity to un-block any images you’d like to see in any of your emails, but will be better protected from spammers by this precaution.
6. Above the options area, click Save Changes.
You have other options, which you’ll see in this list – to show all images, or to show images only from your contacts and from certified senders.
Also, with “Initially block all images” activated, you can still selectively turn ON images on an email-by-email basis by clicking Show Images at the top of any message that contains images.
Not sure about Hotmail though
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Disable image on hotmail and yahoomail 
