The value of this application is often overlooked by Mac users, perhaps because of its unassuming name.
Among many other uses Preview is good as a cropping device:
1. Open graphic file in Preview
2. Under Tools choose Select Tool
3. Drag the cross cursor over the part of the image to select what you want
4. Under Edit - copy
5. Open/go back to your Pages file and paste.
Then convert your Pages document to PDF. You may find it also helps to reduce the size of your files.
I've tried this workaround many times and find it easier and quicker than masking, especially when you have to deal with large format graphic files, from which you only need to use a small fragment.