As you stated yourself, your email service began blocking jpg attachments from specific domains. If they're blocking them, you won't SEE them in the web browser, period. Besides, web based email is not a "program". It is simply another web page being displayed in your browser which is at the mercy of the programming which creates the pages. If the email service has blocked certain things from being displayed, there's nothing you can do about it except to switch to another web based email service providers.
The attachments are still there in the email though. Since Outlook Express isn't smart enough to block them, it will allow you to download the attachments and open them. Of course, this behavior will eventually help you catch a nasty virus. I suggest you take the advice of the email service and not open them because many anti-virus/anti-spyware/anti-trojan programs aren't capable of detecting some of the new viruses embedded in JPG files.