You can start working from anything!
I often use regular photo's as input, things I encounter online or sometimes pictures from friends on facebook.
Sometimes I put a lot (like, a lot) of time in a single work or image, but that is not always necessary. I also make stuff to relax sometimes, or to try out a new technique.
Take for example the picture below:
I made this based on an image I encountered on reddit, showing a man with strong sense of style reading a book on the subway:
The technique in this case couldn't be easier: I literally just drew selections in layers above the base image and filled them with colors. Because they were different layers, I could shift them in position and move their respective hue, saturation and transparency until I was satisfied with the visual result.
Another example:
This picture I created from a summer photo from a friend:
The idea was to reduce the background to its simplest color elements and retain the sense of summer.
I'd say that is the common element in most of my art, that I'm trying to emphasize a certain impression, the feeling that I get from the image in the first place.
An example (this one took me longer than the preceding two) is the following image, that I took on a rainy day:
I felt the image was more beautiful in real life, so I altered the image to give the wet, urban green impression that it gave me watching it (which is technically less realistic)
To me, that's one of the reasons why I think art is also about sharing ideas or viewpoints. You get an image in your head that strikes you as beautiful. By making it real, you can show it to others and ask them: do you feel the same?
Feel free to let me know!