When using images for non-educational work, there are rules in place to protect the creator of the original image. The best thing to do is to find images through Creative Commons that allow modification.
One of the most useful tools for image-related scholarly research is Artstor. However, be mindful of copyright restrictions - you can use them in your homework, presentations, password-protected education websites (Canvas), and personal use (inspiration) but you must research copyright permissions for each image for public use/publications.
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