PPTX files are really zip files in disguise.
Some unzip programs can directly unzip pptx files others you need to change the pptx to zip.
We recommend 7Zip which is free and very good
http://www.7-zip.org/
(In house we have pro software to change the zip contents but you would need to spend a fair amount of money)
Once unzipped you can see the folder structure and the images will be in the PPT\Media folder.
You CAN change the images and rezip but I would just modify them and reset the backgrounds in your template (Right click a blank part of the layout > Format Background)