Don't add '@2x' to imageURLString if image filename already has it

Not sure if this an iOS issue or more how you guys upload the image on the server side for the In-app notifications.

The problem is that we have uploaded an image which filename contained "@2x" already so you were adding another "@2x" to it. Then when trying to fetch it from the server, it was throwing an error:
`"image failed to load from URL ..."`

Note that this fix is just a patch so the notification shows up but the image will show its small version. 
This could be the reflection of an underlying issue.