Appears normally in my iPhone 6s, iOS 13.4.1. Perhaps there is a setting in the app to force the use of "Dark" mode? Or, in Settings->Display & Brightness, you have "Dark" mode
only enabled? Or the switching is set, in Options, to a custom schedule?
BTW, did you know you can reduce the size of an image without needing any graphics app? Simply open the image in Preview (control-click, if needed).
Select->Tools->Adjust Size.