That's a feature of Windows Picture and Fax Viewer (WPFV).  UI presume
that's what is displaying the images.

When you open any attachment within Outlook Express (OE) it saves a
temporary copy in the Internet Explorer Temporary Internet Files (TIF),
then launches the appropriate program (based on the Windows File
Associations) passing the name of the saved attachment.

Once the WPFV is started, the next and previous buttons will cycle
through any other files in the same folder that it can handle.  The same
thing will happen if you double click any image in any other folder on
your PC.  WPFV will cycle through all the pictures in the folder.

As for the TIF, its primary function is to be a cache of everything that
Internet Explorer (IE) needs to render a web page.  So pretty much
everything IE handles (HTML, images, non-steaming videos, non-streaming
audio, etc.) will get downloaded into the TIF before IE renders it.  It
stays there until it's eventually purged on a first-in-first-out basis.

So once you click the next/prior buttons, you aren't see what came in
the OE message, but rather any recent images that were displayed by IE.