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You can always download pics from internet provided you do not use copyright protected images. But I will suggest you to go out, look around yourself, take out your phone's camera (or if you have a point & shoot, that will be even better), and just click a pic that you find will compliment the essence of your blog and then post the same.
It is always suggested to make your own pictures and images. If you can't they look for some agency to provide you with. And if you're looking for free stuff, Google is the king. But never use copyright protected images, someone will find you and hunt you down (metaphorically) :)
You can get free high-quality images at https://pixabay.com/
and http://www.freedigitalphotos.net/
Either from their own devices/cameras or royalty-free stock images.However, other images are usable provided proper referencing and acknowledgments are provided.
Please before applying any picture to your post verify yourself that your are not about to break copyright law. bloggers can get pictures related to their post from different sites where it is not illegal or copyright law to share or download. try search about free images related to your posts, note that there are websites who sell pictures.