HOW-TO: Image Preparation

[kuroi]

I know there is a plugin named IM, but I have not applied it successfully except under the classic theme!

Oh, sorry, it might be IH2, is it right?

Extract from the Image Handler readme file

If you use a custom template, copy the contents of
includes/templates/templates_custom into your custom template’s folder or
merge the files if the target files have already been customized by you.

This is already done for classic, which is why that works for you.

There is a long and detailed support thread for Image Handler where any further questions on that mod should be posted.