Rainmeter is "open source". The skins and suites that others create with it absolutely ARE NOT.
It is both true and a good thing that the Rainmeter community is one where everyone learns from everyone else, and putting out a skin in public means you are at once offering that skin for users to enjoy, and for others to tear apart and learn from. This is a big part of why Rainmeter is so popular. The open nature of the skin code (open in the sense of readable, not locked down or compiled) really reduces the "price of admission" to Rainmeter for new authors.
Every single one of us, without exception, benefited from messing with other folk's skins.
However, skins and graphics and addons folks do for their skins are still their intellectual property. Depending on the type of licence they put in the Metadata of the skin, or any restrictions on use they specify in comments or readme files, they may intend for all or part of their work to be freely used by others, or not. That is their absolute right.
It is at once the right thing to do, and in the best interests of the community, for everyone to pay attention and abide by how an author licenses their work. It's not rocket science folks. If an author says "use this as you see fit", then go for it. If there are restrictions like "non-commercial" or "share and share alike" or "no derivatives", then either contact the author for permission, or just don't use the other person's work in a way they have restricted.
http://docs.rainmeter.net/manual/publishing-skins#GuidelineLicense
At ALL times we should be providing credit when we use other people's work. Skin code, graphics, addons, any part of someone else's hard work. That should be done no matter what kind of license is on the skin. Period. Link to the original work when you post your skin, and make it clear that while you may have used someone else's ideas in a new and unique way, you are obligated to them for the original creation.
We need to make a practice of "shaming" folks who don't have any regard for where the credit lies for things they put out. I particularly hate those "compilation" skin suites that constantly pop up that are nothing more than a bunch of other people's skins, set on a different wallpaper (also used without attribution most of the time) and posted on deviantART or Custo as "My Wonderful Creation". No attribution, no links to the original work. I would like to see pointed comments on those submissions every time.
The nature of Rainmeter means you are not going to be able to protect your work in any real way once you post it somewhere. Lots of people are going to steal your graphics, use your skins or skin code without attribution, and generally ignore any license you put on your stuff. While this can be very annoying, I still think on balance it is really a price that we pay for the open nature of the way Rainmeter works and the large and dynamic community that surrounds Rainmeter.
My advice is to comment on anything you see posted that uses your work, asking them to provide credit and attribution. After that, let it go. Trying to police this, even for just your own work, is a losing battle and one that will just drive you crazy. It is just human nature that some number of folks are never going to understand or care what the right way to benefit from other's work is.