you can detect the walls by creating a custom arfragment and making it detect planes vertically then you can place anchors on the wall . Another option will be using markers on the wall

hi i want to create some thing like this
picture that

but i do not know how to place anchors on a wall and place images on the wall using those anchors

 AR fragment
fragment = (ArFragment) 
getSupportFragmentManager().findFragmentById(R.id.sceneform_fragment);

CompletableFuture<ViewRenderable> imgView = ViewRenderable.builder()
        .setView(this, R.layout.textview)
        .build();

CompletableFuture.allOf(imgView).handle((notUsed, throwable) -> {
    // When you build a Renderable, Sceneform loads its resources in the background while
    // returning a CompletableFuture. Call handle(), thenAccept(), or check isDone()
    // before calling get().

    if (throwable != null) {

        Log.d("MainActivity", "onCreate: " + "Unable to load renderable");
        return null;
    }

    try {

        imageViewRenderable = imgView.get();

    } catch (InterruptedException | ExecutionException ex) {
        Log.d("MainActivity", "onCreate: Unable to load renderable");

    }

    return null;
});

try {
    mArSession = new Session(this);
} catch (UnavailableArcoreNotInstalledException e) {
    e.printStackTrace();
} catch (UnavailableApkTooOldException e) {
    e.printStackTrace();
} catch (UnavailableSdkTooOldException e) {
    e.printStackTrace();
} catch (UnavailableDeviceNotCompatibleException e) {
    e.printStackTrace();
}

mArConfig = new Config(mArSession);
mArConfig.setUpdateMode(Config.UpdateMode.LATEST_CAMERA_IMAGE);
mArConfig.setPlaneFindingMode(Config.PlaneFindingMode.VERTICAL);
mArSession.configure(mArConfig);

fragment.getArSceneView().setupSession(mArSession);

fragment.setOnTapArPlaneListener((HitResult hitresult, Plane plane, MotionEvent motionevent) -> {
            Anchor anchor = hitresult.createAnchor();
            AnchorNode anchorNode = new AnchorNode(anchor);

            anchorNode.setParent(fragment.getArSceneView().getScene());
            TransformableNode art = new TransformableNode(fragment.getTransformationSystem());
            art.setParent(anchorNode);
            art.setRenderable(imageViewRenderable);
            art.select();

            ImageView imageView = (ImageView)imageViewRenderable.getView();
            Picasso.get().load("http://www.cbk-enschede.nl/plaatjes/ens/jpg/GSA023.jpg").into(imageView);

        }
);

this is the code that i got from a question on stackoverflow this is the one i want my user to be able to add images from gallery to the wall