Load image from Pipe : Angular 4

Question

.html

<span *ngFor="let ydata of Data">
    <!-- some html -->
    <img fxFlex="85px" class="image" [src]="ydata.attachments[0].thumbnail_url | getImage>
</span>    

.pipe.ts

import { Injectable, Pipe, PipeTransform } from '@angular/core';
import { DomSanitizer } from '@angular/platform-browser';
import { HomeService } from '../services/home/home.service';

@Pipe({ name: 'getImage' })
export class GetImagePipe implements PipeTransform {
    constructor(private _homeService: HomeService, private _sanitizer:DomSanitizer) {
    }
    transform(url: any): any {
        // this return is working
        // return '<somebase base64 string>'
        this._homeService.getImg(url).subscribe(
            res => {
                return res._body;
            },
            error => {
                return 'error';
            }
        );
    }
}    

not getting any response from pipe. I can see the service call on network and its getting base64 image string as response from service. But, in html its not updating always throwing as null.