For reference-based image workflows, start with a clear source image at a useful resolution. The subject should be easy to see and not heavily covered by blur, text, filters or foreground objects.
For people
A natural, well-lit face with enough detail usually gives the model more information to work with. If a workflow accepts multiple references, use images that are consistent with the identity or look you want to preserve.
For products or clothing
Use a clean image that clearly shows shape, texture, color and important details. Generated results can still vary, so compare outputs with the original before publishing.