There are a few ways that you can edit pictures on Android Gallery. One way is to use the built-in editing tool. To do this, open the Gallery app and select the picture that you want to edit. Then, tap on the edit icon (it looks like a pencil). This will open the editing tool. From here, you can crop, rotate, or adjust the brightness, contrast, and saturation of the picture.
Another way to edit pictures on Android Gallery is to download a third-party editing app from the Google Play Store. There are many different editing apps available, so find one that suits your needs. Once you have installed the app, open it and select the picture that you want to edit. Then, follow the instructions on how to edit the picture within the app.