KNOW YOUR RIGHTS
When using images on the internet, it's important to know the particular restrictions for each classification. Protected images are the most exclusive; you're essentially renting it from the original author. For a fee, you are the only one who can use the image for an arranged period of time. Royalty free images also require payment, but multiple users can buy the same one, meaning you can be purchasing duplicate content. Most likely you're looking for public domain images which are free to use however you want. This composes a good amount of the photos out there and is most likely what smaller blogs are looking for.
OUR OLD PHOTOS
One option is morguefile.com. At morgueFile you can search through thousands of pictures that can be used for your own creative work, whether it is commercial or not. The site works as an image sharing hub, where users will take other's images for free and then upload their own in return. A morgue file in the newspaper business is a place to keep post production materials for use of reference. Similarly, MorgueFile provides free image references for the entire web. We at A Small Orange have used a good number of these images in our articles.
If you like the feeling of an online image sharing community, then Flickr.com may be for you. On Flickr you have the opportunity to store, search, and sort your images as well as upload them. You can set privacy settings on your photos or share them with the world and see what comments or tags people make. Your level of involvement is up to you. Flickr is one of the biggest sharing communities, but there are others like Photobucket.com or imgur.com. Many use Flickr due to its popularity, which means there are more pictures available and a stronger community.
PHOTOS ON DEMAND
If you've searched on morgueFile, Flickr and Photobucket and just can't seem to find exactly what you're looking for, there's another option. On PhotoRogue.com you can browse through available pictures, but the main feature is a requesting option. A user will post what kind of photo they're looking for (the most recent was a spider web covered in rain) and volunteer photographers will upload their pictures in response so that there are 6-8 options to pick from. Since this process does take some time, it may not work for the blogger/writer who is in a hurry. However, it does give you several good options for very specific material.
A FEW RULES
Most of these sites have a fairly open licensing agreement, but there are a few conditions. You are more than allowed to adapt and change the images to your project. Your project can be either personal or commercial work and you don't have to give any attribution to the original author. However, you may not claim ownership of the image in its original state. Also you may not sell, license, sublicense, rent, or transfer this image in its original condition without alteration. For the most part, as long as you're not claiming the image as your own or trying to directly make profit, you should be fine.
IN YOUR search for relevant images and material, you have multiple options at your disposal. You could join the masses and use Google image search, but that will most likely give you duplicate content, and put you at risk of copyright violation. If you're hoping to use previously used content that is free domain then try morgueFile. If you want to participate in a bigger image sharing community, then try Flickr or Photobucket. Or try something new like PhotoRogue and get very specific content if you have the time. Ultimately, trying multiple options is the way to go but as long as you're willing to work and have an open mind you'll find something good.