The best way to find a free app to manipulate photos while working on a Chromebook would be to look for an extension to download in the Chrome Web Store. This is because the Chrome Web Store offers a wide range of extensions and apps that are specifically optimized for Chrome OS, ensuring compatibility and ease of use.
You could also consider searching for options on Google, but the Chrome Web Store is likely to yield the most relevant results for your specific needs on a Chromebook. Downloading from the internet may pose security risks, and checking your phone for an app would not be as effective since you are looking specifically for a solution that works on your Chromebook.