Why is my floating action button not displaying its image properly?

(Assuming you are using Android Studio) -- Rather than using your own custom .png for the FloatingActionButton's src, create a new vector asset with that icon. Right click your res/drawable folder in Android Studio, and then click to New -> Vector Asset.

The icon you are using is built in to Android Studio, although you could import your own vector asset if you were doing something more custom. The XML it generates will look like this:

<vector xmlns:android="http://schemas.android.com/apk/res/android"
    android:width="24dp"
    android:height="24dp"
    android:viewportWidth="24.0"
    android:viewportHeight="24.0">
<path
    android:fillColor="#FF000000"
    android:pathData="M19,13h-6v6h-2v-6H5v-2h6V5h2v6h6v2z"/>

and you can change the color in fillColor. Set your FloatingActionButton to this drawable in android:src and you're good to go.