The IMG tag can be easily abused and track and log visitor IPs; present a potential security risk for visitors.

IMG tags would certainly save much bandwidth, but would greatly increase latency - more specifically, annoying pauses while the visitor's web browser resolves the domain, when applicable, of each linked image, establishes a connection to each resource, and retreives the data, potentially from various far-flung servers / networks; defeats the benefits of pipelining and other various speed enhancing tweaks.

In a nutshell, IMG tag is purposedly disabled for a better visitor experience - not allowing IMG tag eliminates many latency issues, enhances privacy / security, etc.

Ron