Several file types are typically used in the graphics industry. They can be divided into two general categories; raster and vector.
Rasterized imagery contains individual colored pixels to build an image. This makes the image easy to process and display on web browsers and mobile devices, but depending on the resolution of the image (how many colored pixels make up the image), it can become jagged and pixelated when zoomed-in, resized, or reprocessed too often.
If you use rasterized images in your posters, make sure you acquire the highest resolution image you are able to in order to minimize pixelation. Images processed for use on the Internet are usually only 72 DPI (dots per inch), which results in smaller files and while maintaining their quality. If you are looking for images for printing purposes, look for images that are around 300 DPI, since printed posters and documents show higher details.
Raster Image Types
JPG is most often used for photographs and images on the web. These files are usually optimized to be as small as possible while still looking sharp and non-pixelated. JPGs can not have transparent backgrounds; they always have a colored background.
PNG is similar to JPG with two primary differences: it is generally larger in file size and has better image quality than JPG, and they can have a transparent background. Therefore a PNG is ideal for saving logo files for websites or irregularly shaped image subjects because they can blend in over colored backgrounds.
TIF are large raster images that, if created properly, are lossless (do not loose pixel information) and can be edited over and over without degradation. They are typically the largest of the image formats but also are the most true to the original image. These is one of the best formats for creating clear, large image publications.
GIF is another raster image type which is more compressible, and smaller, but displays less colors than other formats. Their best use in posters are for small, simple illustrations with solid colors.
Vector Images use mathematical formulas to construct images. This allows for more flexibility in scaling since each part of the image is based on a coordinate system and helps avoid pixelization. Adobe products tend to focus on vector images.
Vector Image Types
SVG is the most common of the vector formats. They usually take up much less space than rasterized images, and can scale easily. They are not suited for photorealistic images and photographs but can display a large amount of detail for illustrations.
AI is a proprietary vector file type created by Adobe that can only be created or edited with Adobe Illustrator. It is most commonly used for creating logos, illustrations and print layouts. You will have to convert this file into a SVG or rasterized image in order to use it in programs like PowerPoint or Publisher.
"VectorBitmapExample" by Original uploader was Darth Stabro at en.wikipedia - Transferred from en.wikipedia; transferred to Commons by User:Pbroks13 using CommonsHelper.. Licensed under CC BY-SA 3.0 via Wikimedia Commons.
Consider the resources below for raster images. Make sure to attribute all images you use.