Start with a clear subject
Photos with one obvious subject, even lighting, and a simple background convert more cleanly than busy snapshots. Crop away unrelated edges first, then raise contrast so the important silhouette is easy to distinguish from the background.
Simplify rather than trace every detail
Use an editor that can detect edges or reduce the image to a high-contrast sketch. Remove tiny texture, duplicate lines, and background noise by hand. Closed shapes matter more than photographic accuracy: a colorist needs to know where one area ends and another begins.
Check the file before sharing it
Export a black-and-white PNG or PDF, print it at the size people will actually use, and inspect the smallest regions. Keep the original photo and confirm that you have permission to use it, especially for a classroom, business, or public download.