When creating graphic layouts for print, web and other platforms, it’s important to know what the right image format is for each need. Using the wrong type of image can cause problems with quality, delays in production, and often costly fees to re-do a project. You can avoid all of these things by planning in advance and being familiar with the different types of image files and how they are best used. Here is a brief guide for non-designers to understanding the differences between graphic file types.
Image types and resolution
Raster (or bitmap) images are typically photos or scans; they are composed of many individual pixels. Because you cannot increase the number of pixels in an existing image without losing quality, it’s important that images are high enough resolution. Bitmap image resolution is measured in pixels per inch (PPI) or dots per inch (DPI). PPI is technically more correct, but often the terms are used interchangeably.
Photos that will be printed on an offset press should be no less than 250 ppi at the final size they’ll be used in the layout. The final size is important, because if you take a 250 ppi image that is 4 inches wide and double its size so that it’s 8 inches wide, all of those pixels also double in size, and you end up with an image that is only 125 ppi.
Images that will only be viewed online, in presentations or on mobile devices can be of much lower resolution. Images used on websites do not need to be more than 72 ppi, but that number is beginning to change — high-resolution retina screens can display up to 326 ppi.
Vector images are line art that has been drawn in a program such as Adobe Illustrator. Things like logos and graphic illustrations are good candidates for vector art. All vector art is a combination of points, lines and fills — the shapes are manipulated with handles that adjust the shape of the line segments between the points. Because vector images are essentially drawn using mathematical formulas, and not individual pixels, they are resolution independent and can be scaled to any size.
Vector images are usually saved as EPS (Encapsulated Post Script) files, or often as native Adobe Illustrator files, which will have the extension .ai on the file. These can be imported into other graphics programs and scaled to any dimensions needed.
Raster images such as photography can be saved in a variety of formats; the most common for print are TIFF (Tagged Image File Format) and PSD, a native Photoshop format that works well for images with transparency or clipping paths. For web and mobile use, the most used are JPG (also called JPEG, from its origins in the Joint Photographic Experts Group which created the format), GIF (Graphics Interchange Format) and PNG (Portable Network Graphics) format, which allows transparency.
Image files should not be supplied as graphics embedded in Microsoft Word or PowerPoint documents, as they lose quality and are difficult to extract for higher resolution uses.
It’s important to know what your final output methods will be before supplying artwork, so if you’re not sure it’s always best to ask. We here at Williams-Helde can guide you and help choose the best file format at the right resolution at the beginning of a project, which can help save time and money when final files are released to printers or other vendors. If you ever have any questions about the right image format to use, please ask us; we’d love to help!