As a graphic designer for several small magazines, I receive a lot of work from other designers, and it is always surprising to me to find that these designers don't realize what mode their images should be.
When images are downloaded from Dreamstime, they are in RGB mode - meaning red, green, blue. This is great for the web. And it allows for smaller file sizes which is great for download times.
But when designing for a printed publication, images HAVE to be set to CMYK - cyan, magenta, yellow, and black. These are the actual ink colors used on a 4 color press. Applications now will automatically convert images from RGB to CMYK when writing the press-ready files, but the problem is this:
Images that are very bright and vibrant while in RGB can often turn dull or even dark when converted to CMYK.
The image of the butterfly looks nice and bright in RGB, but may turn slightly dull looking once converted to CMYK.
So - as designers creating print publications and photographers selling images directly to clients who plan to print, take the time to look at your images in CMYK mode. If there are noticeable changes, work on color correction. Sometimes the changes cannot be improved upon simply because RGB allows for more color possibilities than CMYK, but you can still make adjustments to keep your images looking the best they can.