How to get inline image in the mail body!Error

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