How to use custom jquery plugins in angular 4

This is because the typings for jquery don't include any reference to imgViewer2, you can patch that in your component source by including this bit:

declare var $: JQuery;

declare global {
  interface JQuery {
    (selector: string): JQuery;
    imgViewer2(): JQuery;
  }
}

This will let typescript know that it is ok to call imgViewer2() as a method on an object of type JQuery (which I assume you already define since you've said you're using jquery in your app)

Add this code in the same file where you want to make the call to imgViewer2, in the global level.

EDIT

To complete this answer, in order to use jquery properly in the application add it to the scripts array inside the file .angular-cli.json thus:

scripts: [
  "../node_modules/jquery/dist/jquery.min.js"
]

Then use the above declaration in your component, here's a sample skeleton component:

import { Component } from '@angular/core';

declare var $: JQuery;

declare global {
  interface JQuery {
    (selector: string): JQuery;
    imgViewer2(): JQuery;
  }
}

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css']
})
export class AppComponent {
  constructor() {
    console.log($);
    $('div#myDiv').imgViewer2();
  }
}

EDIT 2

Alternatively, if you install the jquery typings npm i --save-dev @types/jquery then you can use a somewhat different strategy which is more strongly typed:

import { Component } from '@angular/core';
import 'jquery';

declare global {
  interface JQuery {
    imgViewer2(): JQuery;
  }
}

Note that you now have to import 'jquery', you dont have to declare var $..., and also that you don't have to repeat the call signature, as it is already defined for you in the typings library.