High resolution is fine for the book. (but do your due diligence and run some tests)

The lower resolution is just for Vuforia to create its dataset for how it sees your image.  Seems counterintuitive but feeding the target manger with too much information can actually decrease tracking quality.  

Thank you for your answer I succeeded to reach 5 stars with your advices. You saved me a lot of time ! I added some padding and worked with a width of 1200px.

I have one more question : As I am working on a book with printed designs such as the one I sent, should I print my original design (with a width of 3500px) or the one I uploaded on the target manager ?

 

Well all images are converted to b/w, so color wouldnt help you anyways.  Sometimes 4 star images work really well, have you tried it?

- Use the show features to see tracking points in the Target Manager

Then try this, all the while looking at the features

- Add more detail

- Less symetrical

- Add some border padding so the detail in the edges will be included (edges get cropped off)

- Play with pixel size, 800-1200px width is a good starting point