Across all agencies, you will find the images are available in different sizes, sometimes three or four, sometimes up to seven or eight sizes. What size do you need for your project?
Electronic Uses – Web, Facebook, iPad apps
In general, the first thing to consider is the media you are using the images in: Electronic screens usually need a smaller size to display images than printed media. A classic TV (before the HD era) displayed only 576 (European PAL and SECAM) or 480 (American NTSC) “lines” and we were watching the images just fine, weren't we? Then again, a full size page in a magazine requires about 3.500 points from top to bottom.
Today's screens are getting better in their resolutions with the newest generations of cell phones showing almost the same density of single points (pixels) as printed media. Still a comparably small image will be sufficient to show up well to the user.
Most of the images you are seeing on the internet, along with an article or a product description, will show image sizes of 300 to 400 pixels on their long side, sometimes even 120 to 150 pixels will be enough. If you are just looking for images illustrating a product category or as the lead image to a blog article, at most agencies the smallest size will be just fine.
Size Limitations in image licenses
If you want the image to show bigger on the web, the second-smallest size – usually about 600-900 pixels wide at most agencies – will serve you well. For example, a header image for a Facebook page will need about 900 pixels in width.
Above that size, you will also have to consider the specific license agreements of your agency: Many agencies will have a limitation of the size of their images if used in electronic format. These can be as small as 800×600 pixels, in some cases agencies allow up to 1200 pixels, others maybe 1600 or have no limit at all. Size restrictions are to protect the image rights as anything published on the internet can easily be “stolen” by others by copying them from your website.
Usage of stock images in print products
For printed media, you will usually be asked to provide images with 300dpi for optimal results: Which means depending on the size of your image, you need a resolution of 300 pixels for each inch – or about 120 pixels for each centimeter – you want the image to show on paper. A postcard is about 6 by 4 inches or 15 by 10 centimers. To print an image in postcard size, you will need 6 * 300 = 1800 pixels. If you only need an image for a small corner of your brochure, you will be fine with about 1000 pixels; if you want it as a full page in a magazine, you need to look for about 3.600 pixels on the long side.
If you want to print anything larger than a full page – say you need images for an exhibition booth, a bill board or an A0 / E ad at the bus stop, you should download the largest size available. Though the resolution in most cases will not be sufficient to print the image in 300dpi, those usage forms usually will be printed in lower resolutions and you will be fine.
Test it before you buy
If you are unsure which size you need to buy, try to find a sample image in the free image section of an agency and download the image in different sizes to test the final results before spending real money.
In case you end up with an image too small to be usable, you should contact the agency – while a specialist might upsize an image a bit, it usually will lose in quality. It is better to ask your agency if they have an option to get a higher resolution of the image paying only the difference in price.