A Little On Extensions and How to Find Them: You can find extensions in the Chrome Web Store, which can be found on your apps page of home screen when using Chrome in a few places. As you can see on the right, I have the Web Store on the main page, as well as the ability to access it in the lower right hand corner of the screen. When extensions are added to your Chrome Browser, you can find them as small icons at the end of your address bar (see upper right hand corner of image).
Now Back to the PicMonkey Extension:
When on a page that contains photos (below I'm on Google Images), I can click on the extension icon at the top right. It will then generate a drop down of all the images on that page. By clicking on the small image in the drop down, I am then immediately taken to the PicMonkey website with that image already loaded ready to edit.
As you can see in the image below, the workspace in PicMonkey has a lot of great options. Quickly rotate or crop, add text or stickers, frames, adjust brightness...
As always, make sure that you are using photos correctly, following copyright and fair use guidelines. In Google Images advanced settings, you can adjust the filtering to all for only "free to use or share". We have a post on where to find this option "Free Image Search with Google".