give a try at:
strFilePath = FullFilePathToTheImage
Assign Activity:
strB64 = Convert.ToBase64String(File.ReadAllBytes(strFilePath))
Keep in mind:
strB64 is only the part:
the red part you would handle e.g. by prepending or within the html part
It can fail, when done this the first time. But just focus on the correct red handle part, then it it will work