How can I replace the "gallery" app? (On rooted bluestacks app player)
Most apps, when they want to choose an image, use an implicit intent, which just says what they want to do, and doesn't specifically name the Gallery app. This means that any app that can perform that action can be started, and if you have more than one, you get the chooser to choose which to launch. This is the normal behaviour.

You won't get a chooser if you've previously clicked Always in the chooser to set one app as the default, or if the calling app uses an explicit intent, naming the Gallery app specifically.

It's best to check the first case. In the device settings, go to the list of all apps, and find the Gallery. Click it to go into that app's info page, then click Clear defaults. If Clear defaults is greyed out, then it's not set as the default for anything.

If you still don't get a chooser, then it's the second case: the calling app is specifically asking for the Gallery, not just for any app that can choose an image. Only the author of the calling app can fix this.