There are a number of considerations and caveats in your question, but here is two cents. When you grab an image from Google, you have to determine (1) whether the image is subject to copyright protection and if so, whether your use of the image is permitted by the copyright owner. In your hypo I will assume that the image from Google is subject to protection and is controlled by a copyright owner who has reserved all rights--i.e. your use is entirely unpermitted. In your hypo you would then be making a copy of the image and altering it. This type of manipulation would be a "derivative" work of the original. That is to say, you literally did copy the original but have added or subtracted in such a way that creates a new original work. Your new original work would be subject to its own copyright protection but you still have to worry about whether it infringes the prior work. Your reference to Warhol is relevant because your defense to infringement would rest upon the doctrine of fair use. But to know whether your use is a "fair use" is a complicated question and the best legal opinion you will ever get is a "maybe" or "it depends."
At its essence, the fair use doctrine looks at four factors: (1) the purpose and character of your use; (2) the nature of the copyrighted work; (3) the amount and substantiality of the portions taken; and (4) the effect of the use on the potential market for the original. Since your use is going to be on a commercial article of clothing, your use would be commercial in nature---as opposed to purely artistic like Warhol's use--and this weighs against a fair use. However, your use is transformative in nature in that it took a simple photo and made it into an artistic image. Nevertheless, courts are reluctant to give much weight to transformative uses that do not comment or critique the original or something else (such as parodies). The nature of the copyrighted work is unknown in your hypo so this factor is neutral. The amount and substantiality of the work taken weighs against fair use because you took the entire photo as opposed to just using a small portion of it. And finally, the effect of your use on the potential market is unknown because your hypo does not provide information regarding the market for the original and whether it is also sold on commercial clothing articles. If sued by the copyright owner, a court would analyze these factors and decide whether your use is infringing or a fair use. Now you see why lawyers are reluctant to opine with any certainty.
You also need to consider trademark and right of publicity issues. In general, Warhol got away with making art out of trademarks like Campbell Soup and other previous art because his uses were purely artistic--they were not being used to sell t-shirts at the time and his uses were transformative in nature and commented on the original works, society, or other issues in such a way that they were of significant artistic importance. In my practice I often give this rule of thumb to any artist wanting to know whether their use is "fair." If you are planning on putting it on a single canvas in an art gallery it is probably fine because these types of uses do not affect the market for the original and are purely artistic in character. If you are planning on making t-shirts, putting the images on bags, or printing books your use then starts to look like a commercial venture (not purely artistic) and these types of uses often draw the attention of the original owners and can lead to cease and desist requests or legal action. Of course, it is impossible to opine properly without more details and you cannot rely on this type of information in lieu of legal advice.
Hope that helps.
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The line where you violated copyright protection started when you created a derivative work using an unlicensed image, and it continues from there.
As for Warhol, see the above post.