var logoUrl = "https://my.site.com/path/to/logo.png"
var logoBlob = UrlFetchApp.fetch(logoUrl).getBlob().setName("logo");
...
MailApp.sendEmail({
    to: client.email,
    subject: "...",
    htmlBody: // html with reference to image like this:
              "<img src='cid:logo' width='107' height='160'/>"
    ,
    inlineImages: {
        logo: logoBlob,
    }
});