How can I call method in other component on vue.js 2?

No two components don't have a parent/child relation. They are all connected through the root vue instance. To access the root vue instance just call this.$root and you get the root instance.

....
.then((response) => {
    this.$root.$emit('createImage', item, response)
});

and in the second component make the function that needs to be triggered

...
mounted() {
    this.$root.$on('createImage', (item, response) => {
        // your code goes here
    })
}

It acts more like a socket. The event will be globally available, due to $root.

N.B. adding the vue instance to global window object is a bad practice

If these 2 components are siblings (no parent & child), then one solution is to use event bus.

General idea is to build a global event handler like so: in main.js

window.Event = new Vue();

Then in your first component fire an event:

....
.then((response) => {
     Event.$emit('createImage', item, response)
});

and in second component register a handler for listening to createImage event in mounted() hook:

...
mounted() {
    Event.$on('createImage', (item, response) => {
        // your code goes here
    }
}

You can find more info by reading this turtorial and watching this screen cast.