Creating a new image canvas is now an image toolkit operation

In D7 you could create n empty image canvas using a toolkit specific function like image_gd_create_tmp() for the GD toolkit. In D8 this has become an image toolkit operation like resize and rotate.

Before

$resource = image_gd_create_tmp($image, 400, 300);

Now

$image->createNew(400, 300, 'jpeg');