The resolution of the image, which are set in the app, should correspond exactly to the actual ones and vice versa:
If you want this resolution (186x120px, like the image is shown in your app) you can / should set it to automatic, otherwise you should resize the image (you uploaded in the assets) to the required resolution. For more details see here:
Rule: Resize images to make them not too large and not too small, but rather, “just the right size”.