I looked around the documentation, but didn't found anything that could help me add a picture in a dialogbox. I actually didn't know what should I have been looking for. Furthermore, if it is possible to add pictures in dialog boxes, I suspect that I should use a path file to that image, but I wish to use my mathematica program in any pc, so how can I have that image in my dialog box, regardless of where I use the code. Thank you
A "dialog box" is just another notebook. It can contain and display anything that can be displayed in a notebook.
Import an image, you get a little picture as the output. Just use this as-is. Do you already know how to show text in a dialog? Just copy and paste this image in place of that text in your code.