Pages does not have extensive image manipulation tools. In Pages v5, there are three principal choices for image manipulation:
- Instant Alpha
- Edit Mask
- Shape Image Fill
With the image selected, items 1 and 2 are found on the Format panel's Image tab, and most likely aren't the tools you truly need. With a shape selected, item 3 will be available from the Style tab of the Format panel in the Fill section. You can fill regular, and irregular shapes with a selected image.
The best included tool with OS X for tweaking image content is the Preview application which has lasso, smart lasso, instant alpha, and the ability to add text into an image. The free LibreOffice, which is purposely designed to replace MS Office, has a dedicated graphics (Draw) application within it that you may find beneficial for image manipulation. I also use the OS X native, open source GIMP, and recently purchased Affinity Designer from the OS X App Store. The latter two allow you to do many Photoshop/Illustrator tasks.