use fixed file name to save background image in documents folder

deltachat-ios/Controller/SettingsBackgroundSelectionController.swift

@@ -132,7 +132,7 @@ class SettingsBackgroundSelectionController: UIViewController, MediaPickerDelega
 
     // MARK: MediaPickerDelegate
     func onImageSelected(image: UIImage) {
-        if let pathInDocDir = ImageFormat.saveImage(image: image) {
+        if let pathInDocDir = ImageFormat.saveImage(image: image, name: Constants.backgroundImageName) {
             UserDefaults.standard.set(pathInDocDir, forKey: Constants.Keys.backgroundImageUrl)
             UserDefaults.standard.synchronize()
             backgroundContainer.image = image

deltachat-ios/Helper/Constants.swift

@@ -13,6 +13,7 @@ struct Constants {
         static let backgroundImageUrl = "__BACKGROUND_IMAGE_URL__"
     }
 
+    static let backgroundImageName = "BACKGROUND_IMAGE"
     static let notificationIdentifier = "deltachat-ios-local-notifications"
 
 }