O Powershell é uma ótima solução para isso.
Set-Location "C:\htmlfiles"
# for each html file
gci *.html | % {
# select the number from the filename
$number = $_.BaseName -replace '\D+(\d+)','$1'
# prepare the new image file name
$newImageName = 'image' + $number + '.jpg'
# get the content of the html file and replace the old image name with the new name
(gc $_).Replace('image.jpg',$newImageName) | sc $_
}