In the case of an advertisement you do need an image that will stop viewers from flipping past your ad, but it's still not enough. Attention is really very easy to get. An image of a train wreck or a dead cat will get attention, but it most certainly won't make people want to buy. The image also needs to increase desire for the product or service that you're selling.
If your talking about images for a website, attention doesn't even enter into it. If they're at your website already you don't need to get their attention, you've already got that. They're there for information. You need well executed, pleasing and interesting images that communicate benefits to the viewer. You're selling trust, and images such as these go a long way to providing the information and trust that they require before they'll buy.
