Android UI Design Tip #2. Use Appropriately Formatted Graphics
We've all seen apps that grind to a halt while trying to load some gigantic graphic file that is not only the wrong size, but a less-than-ideal format. The Android platform supports a number of media formats, such as PNG, JPEG, GIF, BMP and WebP (Android 4.0+). PNG is the ideal format for lossless images, while JPEG quality is "tweakable."
Android also supports stretchable graphics with Nine-Patch. Consider using WebP, when available, in place of JPEG, as it may be a more efficient choice when it comes to storage and download sizes. That said, if used as a separate set of images alongside older formats, the overall package size will be larger, offsetting any benefits of using the newer format.