Just as the passing years have carried many literary classics into the “public domain,” so too many historic images are available for your free use.
There are some normal procedures that allow nonprofit bloggers some leeway in having formal permission for the use of images. First of all, there are some images so simple and commonplace as to be essentially valueless. You would never be able to track down their creator even if you devoted the rest of your life to that purpose.
Then there are images that the owners would almost pay you to use. Think in terms of commercial products, like the one pictured above. You may question the good taste of this brewery’s name choice for their chocolate peanut butter porter, but if you use their image, it’s highly unlikely they would complain.
If you are linking within your blog text to an outside resource, it is normally considered “fair use” to replicate an image from that location. It’s possible they could ask you to remove it from your site, but highly unlikely, since you are directing “traffic” to their own site. As long as you aren’t attempting to deceive or gain some kind of profit from the image, you should be on safe ground.
Here are some places to start when seeking a photograph for your post. Some are not copyrighted at all, and others have many images that are governed by the Creative Commons guidelines. You can learn about the various rights that artists on sites like Flickr are retaining at Creative Commons.