Updating a Plotly imshow image from a dropdown menu

The issue is that you're using "img" as the property name, but Plotly's heatmap traces use "z".

import numpy as np
import plotly.express as px
fig = px.imshow(np.random.rand(5,5))
def createdropdown():
    keys = ["A", "B", "C", "D", "E"]
    buttons = []
    for k in keys:
        buttons.append({
            "method": "restyle",
            "label": k,
            "visible": True,
            "args": [
                {
                    # Changed "img" to "z" and wrapped in list 
                    "z": [np.random.rand(5,5)]  
                }, 
                [0]  # Trace index
            ]
        })
    return buttons
updatemenu = [
    {
        "buttons": createdropdown(),
        "direction": "down",
        "showactive": True
    }
]
fig.update_layout(updatemenus=updatemenu)
fig.show(renderer="browser")