bigger images when moving to smaller theme

I’m just in the process to changing to a newer and responsive theme.

Thing is that my current theme has a wider content area and therefore there’s quite a lot of images that will overhang the smaller theme’s content borders … and into the sidebar.

Is there some sort of process that these larger images could be rescaled, to suit the new theme? I hope there is ??

Thanks for any help from anyone as I’m sure I’m not the first with this challenge
Martin