Here's a great article from the Harvard Law School Library that explains the differences between Public Domain, Creative Commons, and traditional Copyright. It also contains links to the best collections of public domain images online.
The best option imho, is Bing. You can search for images using Bing then filter for Public Domain images by clicking on "License" in the menu below the search box and selecting Public Domain. Please note that Google images is a bit of a mine field since it does not allow you to filter for Public Domain images specifically. Even when you do an advanced search for "free to use, share, or modify, even commercially" Google includes Creative Commons images that usually have restrictions on them. Please be careful.
When building palaces, you have three options:
1) Public Domain images: the best option! Any palaces you create with these images can be listed on the store for sale.
2) Creative Commons: the second best option depending on the specific license used. Some licenses limit you to only non-commercial uses, so those you wouldn't be able to include in palaces listed on the store. And be very careful, even images that are allowed for commercial purposes often still require attribution. For those, you would need to attribute the image in the description of your palace. You need to be very careful to read the actual license description when using Creative Commons images.
3) Copyrighted images: these you can only use if you own the copyright yourself or acquired a license for it.
Hope that helps!
Thanks for the feedback. We'll add a video to show you how to upload palaces to the store. In the meantime, here are some written instructions: 1) go to the account tab, 2) find the section for "publisher account" and click the button there, 3) register with Stripe (please note that for security purposes, we don't store or process any of your financial information -- all of that is done by Stripe), 4) choose the palaces you want to upload, set a price for them, and 5) profit!
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