Hi luckytran112233d49v2, welcome to Thunkable! 
Be sure to check out our posts about How to ask Great Questions v2.0, our Community Guidelines, and our Docs as you get started.
The default size of the screen built in Thunkable is 375x667px. If you are wanting the image to render with the same proportions, regardless of screen size, you would want to create a background image with the same aspect ratio and use the screen properties tab to add in a background image. Setting the image’s resize mode to contain will resize the image to fit entirely within the screen while maintaining its aspect ratio.