I would like to create a comic strip, or, rather, a set of comic strips, for posting on social networks such as Facebook or Pinterest.
Your intention sounds good.
How can I start with the following strips and just modify the clouds where they talk?
You cannot start with those strips you quoted, because those are under copyright. You copied even the copyright symbol. And you want to publish, as you mentioned at the start.
"The clouds where they talk" are often called speech balloons.
Every bitmap editor (for example GIMP) can erase the text in those speech balloons (erasor tool or white-paint with any brush) and lets you enter fresh text with the text tool. I am sorry, if you want to "do stuff" you have to learn to use some tools.
I want to create storyboards and would prefer to do this on my mobile phone.
Story boards are normally a substantial collection of quick drawings to help a film-team visualize and plan the filming of a screenplay. Maybe you want to draw cartoons like those examples you show?
If you really prefer to do this on your mobile phone, you might not be in the best forum here. Most users here are using more powerful tools on muchly larger screens. I personally would never even try creating fresh content on a mobile device, not even on a tablet.
The comic strips I am interested in are about patients having sessions with their psychiatrist. I don't care if the comics look simple.
Yes, again, that is a nice idea. Seems that several other artists have already done something similar, which proves that your idea is valid. It will just get harder for you to create unique fresh editions and to avoid the pitfalls of (perceived) plagiarism.
I am undecided whether I should do this on my own.
If it helps you, my personal advice is yes, please do this on your own.
I wonder if it is even possible to have a collection of faces to start with to insert on static bodies and desk to accompany the comic strip.
Yes, it is possible to have templates of faces and work in an assembly-line-style. If you are are great copy writer (text expert), you will get away with visibly industrially created visuals. If you are not super-witty with your dialogs, your results would just look like something done by a lazy person.
Since you do not mind "simple cartoons" I would highyl recommend to draw them fresh for each edition. Might even be less work(!) than to create and mask separate faces and props and then do the admin and assembly.
How should I achieve my goal?
On such an open question I can only pass some little gems like these:
- just start (in your case: grab a pencil and make your first simple
- take a photo with your smart phone and share it here (edit your question) which will motivate the pros to give their input too
- see whether you like the work and the result
- then do another one
- and another
- and sixty more
- now show them to a trusted friend who will not mock you and you will give you loving (but honest) feedback
- stay loose, work with a pencil until you are happy with it
- now maybe trace it with black pen and scan or photograph it
- Pinterest will likely like your stuff more, if it is "really you" than if you just paste face-templates over existing cartoon-raws
- other social-users will give you likes or feedback and you can tweak your style and methods as you get along; people are patient
with beginners, if the stuff if funny or shows potential; you do not
need to be too perfect to begin with