I will start a new blog soon with adsense links. My niche is programming languages and would add an image per blog post. From where i can get images for my blog?
Here's a list of 10 sites with copyright free images that you can use. Since you have mentioned that your niche will be related to programming, if you want images specific to your projects (tutorial-style blog posts), you should also take screenshots of your work.
As long as you are allowed to use those images (in this case the images are released under the Creative Commons license, which allows you to use and edit the images), there should be no problems. Otherwise, that would also include the stock images.
You can use flickr and unsplash to get free images. If you are going to use images with copyrights, its better to credit them properly. Using copyright things is not only against adsense, it's generally not good to do. Why to get into trouble when you have options for free images. As @MariusEnache said you can use screenshots wherever necessary.