From DT Online
Using BIMP you can specify what size and what type the images should be - choose jpeg for photos and gif for diagrams.
A common problem when using PowerPoint, for example, is that users create presentations which are far too big to email or store on the internet. This is because PowerPoint creates the illusion that images are resized to fit but actually stores them at their original (often very large) size.
You can either use something like CorelDraw, PaintShopPro, Picasa or XaraX to modify the images individually before you use them or simply place all images in a folder and use a free download called BIMP to process all images together as a single batch.