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:

Unix

perl -pibak -e 's/src\=\"\'/src\=\'\.\.\//' `find /basepath -name *.ht
+ml`
[download]

Windows

for /f %i in ('dir /s/b d:\basepath | findstr html$') do perl -pibak -
+e 's/src\=\"\'/src\=\'\.\.\//' %i
[download]

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.