ConvertBannerImageUrlToBase64.ps1

# Connect to your site
#  (this example assumes an entry in Windows Credential Manager,
#   but you can pass credentials however you need here)
Connect-PnPOnline https://yourtenant.sharepoint.com/sites/yoursite
$connection = Get-PnPConnection

# Setup a Web Client using credentials pulled from the connection
$client = New-Object System.Net.WebClient
$client.Credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($connection.PSCredential.UserName, $connection.PSCredential.Password)
$client.Headers.Add("X-FORMS_BASED_AUTH_ACCEPTED", "f")

# Get a page
#  (You could be doing this in a loop or using Get-PnPClientSidePage)
$page = Get-PnPListItem -List "Site Pages" -Id 294

# Grab the value of the image and convert it to base-64
#  and slap the data information to the front
$image = "data:image/png;base64," + [convert]::ToBase64String($client.DownloadData($page.FieldValues.BannerImageUrl.Url))

# In this sample, just copying the HTML value to the clipboard to prove it works
#  Normally, you'd build an HTML string and append it for your email
Set-Clipboard -Value "<img src=""$image"">"

# All done!
$client.Dispose()
Disconnect-PnPOnline