1. Read the licenses carefully

Don't use for example textures from cgtextures. They are quite good, but they don't can be released under any opensource license.

2. Don't use resources, which may not be in all countries allowed under this license

I had the issue with picture from old books, which I used as source for icons. The copyright doesn't expire in all countries, so I had to remove them.

3. Do it at your own

Thats the safest way to prevent license issues. If you create everything yourself, there can't be any license issues.

I've made license issues in the past and I want to apologize for them. But I learned from my mistakes and will include in future a list with a detailed list of the sources.