Gregg is wrong. Thanks but no thanks Gregg, you are confusing people. CC-BY-SA allows free commercial use. It just means that if someone makes a derivative of the work, they also keep that derivative CC-BY-SA (which is good) rather than locking down the rights. But they can do commercial use without any limitations.
The use cases shown on the page https://www.pixeltrue.com/frontliner-heroes are all derivative works. BY-SA means you can sell the picture but as so as you add text it's now a new derivative work (like every example on the page)
This is all a bit confusing but it's starting to makes sense now.
What I actually want the license to be is CC-BY-SA - I think this mean that people shouldn't making money out of the illustrations e.g. putting it on a t-shirt and selling it, but rather using it in their commercial website/app.
That just means everyone you share your stuff with must share alike (what SA stands for) anything they choose to publicly publish that they’ve made out of your copylefted work (their derivative works), in the same way you shared it with them (as https://creativecommons.org/licenses/by-nc-sa/3.0/ says: “ShareAlike — If you remix, transform, or build upon the material, you must distribute your contributions under the same license as the original” — in this case CC-BY-SA).
That doesn’t mean they can’t sell it, it just means they can’t claim it as their own and refuse to let any one else make their own derivative works of it under the same terms they made theirs.
There's no license that lets people use it on a commercial website but not to sell it on a shirt.
The only way to have that situation is to keep it All Rights Reserved and make separate contracts with people for all uses. That would totally undermine your goals.
I think CC-BY-SA is what you want. People aren't likely to sell it on a shirt, and if they do, they still have to give you credit (that's the BY part), like on the shirt itself I'd think even. If you left off the -SA part, people could make new versions and make those versions restricted instead of sharing them under the same terms.
Don't listen to this guy, as far as I can tell the only purpose of wanting the share-alike clause gone (which doesn't prevent commercial usage; it literally says that on the page you linked above) is to take your images and sell them on something like shutterstock.
This is wrong because many uses of the work, like adding it on the front page your your site with some caption, is a derivative work. Putting it on a page by itself or printing on a t-shirt would not be derivative but using it as a logo with text would.