What thread should I use if I want it to be as invisible as possible. The reason, I want the picture to come out more than
the stitching.

Choose a color matching the background, or maybe even a varigated with similar colors :D:D:D

Use invisable thread , often refered to as monofilament . It comes in clear , and smoke color( for use on dark fabrics , and does not relect the light as much). I use it often to give texture to an area but do not want the quilting to detract or compete with the patchwork design.
If you choose to use , remember to adjust your top tension. Most machines need the top tension to be loosened for this type of thread.

you could use invisible thread. light gray blends, disappears pretty good too, depending on what you are sewing.

If you don't want to use monofilament, you might try Bottomline from Superior in a neutral color. It is so fine it will not show up very much.

I'm not a particular about the kind of thread,as much as to the color. I've used, polyester, cotton and silk. Color and small stitches.

You could try Invisafil by Wonderfil Threads. It comes in 30 colours and is very thin (like hair).