If you're working with images (as background for example) you could face with big sizes of files.
Something like JPEG file 1280X1920px could take about 0.5 - 1.0M depending of content.
And it increases APK size!
Very nice solution I found is WebP.
You can download one of converters here: xnconvert
In my case I have following result: 789kb -> 96 kb (80% quality)