You can manipulate anything you find on the internet. However, you have to respect the inherent limitations regarding copyright infringement. As long as you do not claim the artwork as your own you, as a courtesy, should ask permission to use the artwork. But again, depends on what you are using the artwork for. Are you using it to show your abilities to make revisions? Or, as a self-training exercise? If so, you can proceed. What you cannot do is claim the artwork as your own, sell it as your own, claim it as your own in your portfolio, etc., etc. The artwork you show were more than likely created using Illustrator, not Photoshop. It is actually more difficult to work these elements in Photoshop. Possible, but not recommended. I suggested using the pen tool to create a path, which would be the same procedure in Illustrator. I am not clear whether you want a white outline around the letters or not. In Photoshop, you could create a Layer for the background color. But, much easier to do in Illustrator with a lot more flexibility. All that said, if you are doing these ops to practice your skill sets, have at it. But, you will have to explain why you chose Photoshop. When I tutor new users, I recommend they perfect their drawing and path manipulation skills using Illustrator, which could then be brought into Photoshop and used when appropriate.
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