It's probably best to use an .htaccess like solution if you can. If you decide to do a find & replace, or are curious as to how this might be done for something else:
perl -pibak -e 's/src\=\"\'/src\=\'\.\.\//' `find /basepath -name *.ht
for /f %i in ('dir /s/b d:\basepath | findstr html$') do perl -pibak -
+e 's/src\=\"\'/src\=\'\.\.\//' %i
This will perform substitution on all html files underneath the specified base directory, and make a backup file (ext bak) for each edited file. You'll probably want to season to taste, and practice on a backup a couple times.