I am writing technical book and wish to use photos from Internet. Not the ones that have watermarks on them, such as these.
The only images you can use are images you have specific permission for to use. This can be because the owner specified. But, there are also a lot of websites with 'Stock' images. These images can be used for commercial and private use. Just don't trust every stock image site you can find, not all of them are legit.
Most of the 'free' stock sites you can find might not be legit, so be sure to check that. There are also a lot of paid stock image sites. This can be paid for an amount of time, or paid per image. These sites are used a lot by professionals and are very trustworthy most of the time.
I could recommend some sites, but I think that would count as ads so I can't post these. Googling will give you some of these too.