Something along the lines of:
<img
insert alt="" src="../Images/example.jpg"/>
-OR-
<img alt="" src="../Images/
insertexample.jpg"/>
Code:
find: <img(.*?)insert(.*?)/>
replace: <hr class="sigil_split_marker" /><img\1\2/><hr class="sigil_split_marker" />
Then (as BeckyEbook said):
click Edit/Split at Markers (F6)