<IMG name="img1" ...> => <IMG name="myImg1" ...>
<IMG ...> => <IMG name="myImg1" ...>

s/<IMG
  ((?:\s+(?!name\b)\w+="[^"]+")*)
  (?:\s+name="[^"]+")?
  ((?:\s+(?!name\b)\w+="[^"]+")*)
  >
 /<IMG name="myName"$1$2>
 /xg;