JPEG - Joint Photographic Experts Group
This is the recommended format for those photo images which must be very small files, for example, for web sites or for email.
PNG- Portable Network Graphics
This is a raster graphics file format (formed of pixels) that supports lossless data compression. This is the recommended format for those photo images which must have transparency. These images are usually larger than JPEGs.
Pixel Resolution or Resolution refers to the total number of pixels that make up an image. It is commonly written as a product of two numbers in the format of M x N, where M is the number of pixels that make up the height and N is the number that make up the width of the image.
Example - 1280x720 -> 1280 refers to height of the images in pixels and 720 refers to the width of the image in pixels.
The values of the resolution will define the orientation of the image (i.e. Landscape or Portrait). If M>N, then the orientation is Landscape as width of the image is greater than its height however if N>M, then the orientation is Portrait as height of the image is greater than its width.
An aspect ratio is defined as the ratio of the width to the height of an image or a screen.
For example: An image of 1920x1080 size has aspect ratio of 1920/1080=16/9.
The most commonly used Aspect Ratios for mobile screens today are 9:16 and 2:3. The images have a Portrait orientation.
To create optimally sized apps, Image files are sliced into the sizes the mobile device can use. This allows small devices to use small images and big devices to only use the big images. These sliced Images are known as Assets.