Professionals sometimes do it and you can too!
Photomanipulation can be amazing! DA images tagged as photomanipulations:
Perhaps the best method is to use your own photos and resources or properly licensed stock images.
A less acceptable method is to copy/paste, collage, trace, or paintover art or photos belonging to somone else, and claim it as your own work. (Please credit contributing artists.)
Here on DA, we see LOTS of young beginner artists, year after year, starting off with photo-manips as a way to produce art that at first glance seems much more advanced than what they could produce from reference or imagination. It wouldn't surprise me if every digital artist has done it at some time or another. I certainly have. Some are casually honest about their methods and they will continue to study and improve. Others will insist that the work is completely their own or merely "referenced" from other images. And it tends to lead to negativity:
1) Inexperienced artist discovers digital art software.
2) Inexperienced artist discovers the joy of photomanip, smudge tool, filters etc.
3) Inexperienced artist whips up a bunch of manips and has a lot of fun making art that, at first glance, seems more advanced than their actual skill level.
4) Inexperienced artist creates attention-seeking threads asking if it's any good or insincerely lamenting how it isn't as good as they would like.
5) Other inexperienced artists OOH and AHHH and the inexperienced artist enjoys the attention.
6) Artists with more experience easily recognize the photomanips as pieces of another artist's art/photos covered with amateurish brush strokes or smudges or filters. Experienced artists mostly roll their eyes and stay silent as they know where this is leading.
7) A few people gently hint that the artist should be clear about the source images and the methods used to make the art.
8) Inexperienced artist does not take the hint. Inexperienced artist deflects the suggestions or admonishments as some kind of flattery.
9) Some of the people who can recognize obvious photomanips become increasingly vocal and critical of the inexperienced artist in order to make the point more clear.
10) Flame war erupts between inexperienced artist, genuinely helpful experienced artists, and trolls.
11) Inexperienced artist generally hurls a few final insults or acts victimized, locks the thread and possibly deactivates account, to come back later with a different name and a more accurate description of their technique.
If you find yourself heading down this path. Just stop commenting, let the thread die, edit your existing posts if you wish, and openly admit that you "collage", "photomanip", or "paintover". It takes some skill to do it well, so be proud of what you do and don't present it as something else.
Good luck and keep working!