Yes its always must to ask the artist before you use their artwork. Some of image you use have some sort of license associated with it. Like for example on devianart has its information page giving details about the copyright.
If no such page is avaliable, try contacting the artist. Most of them will gladly promote their work through your app for free, with the clause that you should mention and link their page.
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是的，在使用艺术家的作品之前总是必须询问他们。您使用的某些图像具有某种与之关联的许可证。例如，devianart 上有其信息页面，提供有关版权的详细信息。
如果没有此类页面，请尝试联系艺术家。他们中的大多数人会很乐意通过您的应用程序免费宣传他们的工作，但您应该提及并链接他们的页面。
Yes its always must to ask the artist before you use their artwork. Some of image you use have some sort of license associated with it. Like for example on devianart has its information page giving details about the copyright.
If no such page is avaliable, try contacting the artist. Most of them will gladly promote their work through your app for free, with the clause that you should mention and link their page.