CNS makes a plugin that will resize pictures. But if you use the FileMaker Inspector in the layout mode, you can tell the picture to resize to the object size in the layout and that will probably take care of it for you.
Under data formatting I have set this to "reduce or enlarge image to fit" and the containers are only small. This however seems to have no effect over resolution of the image and hences still a large file.
If I sent the audit without any images the size is only 112kb, and with two images its 3mb
Using the new container-functions and scripting, you can SET another container's content to be a specific size using a script and the container sizing function. This can give you the optimised image size you want on any record from any size of source image. Use the second container in your output, and your PDFs will always use the same size.
Dig into the new functions in the calc engine for containers, and script the Set Field process to use these new calc values to create the size image you want in the second container field.