In every Android project there is a drawables folder. This folder is where image assets are uploaded to. The images in this folder are packaged with the apk when it is built. The android app can access these images offline and at a much higher speed.
Here is a video with a simple example of adding an image to the drawable folder and then displaying it in an imageview.
As mentioned above, accounting for multiple screen resolutions plays a role in both performance and overall user experience.
Luckily though, Android has a simple way to handle this.
You can read more about it in the documentation.
The size and quantity of the image assets directly affects the size of the apk file.
When adding image assets to the drawable folder, the names of the images are not allowed to contain capital letters or spaces between the words.
Example: Adding the following files will provide the following results
cars.jpg toyota_corolla.png
Cars.png Toyotal Corolla.png Cars!.jpg