Currently Being ModeratedJan 30, 2013 8:48 AM (in response to kirko)
Select the image in Pages. Use the Metrics inspector to resize your image. You can decide to constrain the proportions or not. Make a note of the image size. Repeat for other graphics. You can use the graphics inspector to regulate opacity. From View > Customize Toolbar... you drag the Adjust Image item and drop it on the Pages toolbar. It will allow you to further tweak your image characteristics.
If you are sensitive to document size, you can open your individual images within Preview and from the Tools menubar, select Adjust Size... and you can also see the graphic size change as you scale the image down.
Currently Being ModeratedJan 30, 2013 9:43 AM (in response to kirko)
Bring your first picture into pages and adjust it to the size you want them all to be. Then, with it still selected go to Format Menu > Advanced > Define as Media Placeholder. Now you can duplicate that placeholder (⌘ D, or Edit menu > Duplicate, or Option drag) and place the copies where you want the other pictures. Dragging and dropping a different picture to each Media Placeholder will automatically resize the others to match the original