Fix thumbnailing of images to same size as source

Ghost User
2019-05-23T13:26:20+02:00

Closes #873

#### Previous behaviour
Steps to reproduce:

1. Upload a 2000x430 image as slider
2. Cannot create thumbnail

#### New behaviour
Steps to validate that it works:

1. Upload a 2000x430 image as slider
2. Works

Closes #873 (closed)

Previous behaviour

Steps to reproduce:

1. Upload a 2000x430 image as slider

2. Cannot create thumbnail

New behaviour

Steps to validate that it works:

1. Upload a 2000x430 image as slider

2. Works