Seems to me like you're trying to ask a few different questions under the all-encompassing quoted word "ethically".
And instead of putting a word in quotes, maybe we could list the actual words as they mean to be more specific.
a) Legally, it's understood that NO PICTURE, IMAGE or basic thing can be used without permission. The default answer is always NO. Most swap the legal aspect for 'what they can get away with or How would anyone know? So if it doesn't say, the answer is NO. If you're not sure, the answer is NO.
b) For things that are usable, there is a (commercial) license or Royalty Free or Public Domain usually clearly stated. If not, see a) above.
c) Ethically or morally is probably left to the phrase Artistically as in, how much do you need to do for something to be truly yours? Or how much work, or how many original ideas or changes does it take for tools, resources and assets to become art.....etc. Believe it or not, *most things actually do list an expected amount of end-user creativity or effort. That's usually in the agreement, license or stated that you just can't re-upload or host/sell the thing (as it is) somewhere else.
Morally, it's up the individual. No one can set for someone else what it takes to be a (pure) artist. That's a rabbit hole and there will always be someone doing it more organically, with less help, aid or assistance. Every artist needs to decide for themselves how much they need to do to have their art speak with their own voice.
I will tell that- that amount usually grows over time.
The other 3 considerations are:
Peers: people who know HOW and WHAT you did and can judge your efforts as insiders.
Critics: People who know your work in context and will judge based on how you compare with other efforts.
Audience: Who will ultimately judge by effectiveness. A simple yes, no, don't care, do care.
Where anyone is-at - or how they feel about this will most likely change over time.