How can I edit slide designs in powerpoint? because I want to make some portions of the slide to be light, so when I typed a word there, the audience can read it.

Can I get the picture used by the slide and edit it with my image editor?

right click the slide and go to slide design click customise/edit slide design

Yes. Right click on the background image (if its an image in the Master template, get to it by View - Master) and selecy copy, then paste it into an image editor. Alter it, delete the image out of the (master?) slide and replace it with your altered one.