Html label img src attr is restricted?

digraph structs {
node [shape=none];

struct1 [label=<<TABLE>
  <TR><TD><IMG SRC="local_image.jpg"/></TD></TR>


can work!
but when SRC is a url address, ONLY blank, i have no idea.

Thanks for your help.

You are correct, it does not work - intentionally. For more info, see this request for the same feature: urls for image (#1664) · Issues · graphviz / graphviz · GitLab

You may be interested in add a pre-processor script for downloading external images (!3165) · Merge requests · graphviz / graphviz · GitLab.