I'm clutching at straws here but things to maybe consider..
Are you using a public DNS system like OpenDNS/ Google DNS? maybe this is your bottleneck?
Maybe your AV scanner maybe having a impact on the processing of emails.
What are your span settings like? too restrictive / strict?
companies promoting their goods...
Is the slow to download image problem from a specific business or domain/URL?
I'm thinking it could be due to certain keywords included within the emails which have increased its spam score, thus, processing time?
Have you tried to test if its the Email client itself which is the problem, have you tried to download the email using another email client (Thunderbird /Outlook, Not Outlook Express
You didn't mention if you were using POP or IMAP to retrieve your messages? maybe some issue with the remote server routing / ports??
What device are you using to retrieve your emails ? Laptop / PC / Hand held device like a mobile or Smart device?
I assume its a desktop/Laptop as you mentioned OE, meaning Outlook Express, so what about trying to repeat the test using one of the aforementioned devices to see if the problem still persists.
Are you using any secure encryption to retrieve your mails? this could be causing some latency issues
If so, are you using standard or non-standard secure ports?
* Have your Secure certs expired?
It may sound pointless but have you tried to email yourself a Test message (Incl. an image) to see if there is a noticeable difference between your test email
and a regular email
, this could help to identify restrictive caching for specific file types (*.jpeg *.png, *.gif, etc)
Try sending test email with an image attachment and then test and then try sending test email as an embedded image and test? MIME settings could be configured incorrectly?
Like I said I'm clutching at straws here but maybe worth a look over some of the points above.