Sure, you've heard of Google Images. But have you heard of the resources below? Just remember to properly attribute any images that appear on the poster, even your own!
European Biomass Conference &Exhibition, "_LMM8803" - licensed under CC BY-NC-ND 2.0.
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 largest 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.
JPG (or JPEG) 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. JPG's can not have transparent backgrounds, so will always have a square- or rectangle- shaped colored background.
PNG is similar to JPG with two primary differences--a PNG is generally larger in file size and has better image quality; 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.
TIFs (or TIFF) 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.
A 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 imagery uses mathematical formulas to construct images. This allows for more flexibility in scaling since each part of the image is based on a coordinate system. Master images should always be created in raster format since they never suffer from resolution pixelization. Adobe products tend to focus on vector images.
SVGs are 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.
An AI file 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.