You would need to use a field coded along the lines of:
{IF{MERGEFIELD "Image_URL"}<> "" {INCLUDEPICTURE {IF TRUE "{MERGEFIELD "Image_URL"}"} \d}}

Note: The field brace pairs (i.e. '{ }') for the above example are all created in the document itself, via Ctrl-F9 (Cmd-F9 on a Mac or, if you’re using a laptop, you might need to use Ctrl-Fn-F9); you can't simply type them or copy & paste them from this message. Nor is it practical to add them via any of the standard Word dialogues. The spaces represented in the field constructions are all required.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]