This thread is so useful. I just had no idea and assumed all photos there were fair game, and I use Pixabay a whole lot.
Yeah, think of it this way: Photographs, graphics, art, logos, and fonts are all the work of other people. To say that any of them were fair game to use would be like someone saying that our books are fair game for anyone to use/read/print/give away. Obviously that's not the case. And whether we give away the book for free or not is up to us (the creator and owner), not a random person who happened to see it.
Please understand that I'm not trying to "pick on" you or anything, just a general thought for everybody.
The reason I wanted to write this post is because I see a lot of people (I'm not saying on here, just in general) who think nothing of using a commercial font without paying for it, or using stock photos without buying them (so they're either low-resolution or still have watermarks), or just grabbing an illustration that they liked without contacting the artist. Hell, it's not just individuals; when I was in Vietnam for business, I saw a game show that was on national TV that used a tune as their opening music from Audiojungle and they didn't even pay for it. Why do I know this? Because when the music played, I could hear a robotic voice repeat "Audiojungle" over and over every few seconds -- this is an audio watermark, and it is a clear indicator that they downloaded the Audiojungle sample and used that without authorization, instead of purchasing the right to use the music.
I guess what I'm trying to say is, we should consider the things we're doing when we take other people's work to use for commercial purposes. Are these things given freely? If yes, are we sure it's actually the owner? If yes, do we want to risk them changing their mind and coming after us? It's not like we're using the image or music or font for a children's concert at school, which they may (MAY) forgive us for. We're using those things for commercial purposes. Even if we were in the right that yes those were given freely and yes they were from the owner, we still have the headache to deal with if and when they change their mind.
At the end of the day, I find that using things that are paid for from legitimate sites (whether stock photos, fonts, whatever else) to be the best option for one's comfort and peace of mind. Also, better save receipts and such just in case.