Allow me to drop a little knowledge on ya:
1st: Just because you are not claiming ownership of the photos you are posting, does not make it any less illegal.
2nd: Regardless of whether you are making a profit, it’s still illegal.
As a professional photographer, I deal with image theft CONSTANTLY. You may not see it as a big deal, but by discrediting the photographers or owners of the photos, you are preventing them from getting work. You may not think you are stealing anything or any monetary value but you are.
Luckily for you, Tumblr’s terms of service makes light of copyright theft and if you are slapped with a take down notice, they make it so you can get away with so much before your account is suspended. Tumblr doesn’t want it’s users legally responsible for their lack of copyright law knowledge seeing as most users are not using stolen images for gain (other than for followers in your case.) Tumblr’s TOS also does as much as it can to protect copyright holders. If it did not, we would have to send cease and desist letters or get lawyers involved in the removal of the stolen images. That is if the photog ever even finds their images have been stolen.
Now given, you have not stolen any of my images (yet,) I have had my images stolen tens of thousands of times. You know how much exposure that is that can land a professional photographer paying jobs? Our photos put food on our tables. Some of my followers personally know the struggle (mine personally as well.) The professional photos you use to promote your blog, are not free. By crediting the source, many photographers will agree to that as payment for useage as long as it is for non-commercial usage.
So in short, posting professional photos to promote your blog, is illegal, disrespectful, and not okay. It is punishable by Tumblr and further by law.
You have a nice evening!
I would like to add a side note : Photographers MUST MUST watermark their photos!
Dont give your stuff away anonymously. Sign your work like any other artist. I much rather post the original photo with watermark included, than a cropped version ( yeah some assholes will still steal it) but it will always be a lesser version.
AND give it right back. Take out the shameless self promo assholes always leave in their posts to link back to them. Scum.