It will be good to gain the original photographer for permission before using the photos.
Whether to include the name or not, you ask them first because some don't put up their name as to hide their identity.
It's good that you asked. Photographs and images are intellectual property. If you didn't create the images, they don't belong to you. If it doesn't belong to you, you'll need to ask for permission before saving or using it in any way.
When asking for permission, be upfront about
- specifically which image you want to use
- why you want to use the image
- how you will use the images
- for how long you will use the image
- any commercial gain
- how you will credit the photographer
An example of how to credit would be:
Image (c) Photographer's name. Source: Yahoo.com (if the source was not the photographer). Used with permission.
Seeking permission and crediting a copyright owner is not just an issue of ethics, it helps build your own professional reputation as a photographer/writer/designer/insert your profession here.
I think people, generally, in the spirit of sharing, would grant permissions, for as long as their property is not abused or misused. Don't be disheartened if you come across those who decline. That's because value in such property is, afterall, subjective.