If the image has neither ID nor title nor description nor source, then it does not get a caption at all:
which renders as:
If the image does not have an ID nor title, then it gets an automatically generated ID, just like every other OurBigBook output HTML element, and it is possible for readers to link to that ID on the rendered version, e.g. as:
Note that the 123 is not linked to the Figure <number>., but just a sequential ID that runs over all elements.
This type of ID is of course not stable across document revisions however, since if an image is added before that one, the link will break. So give an ID or title for anything that you expect users to link to.
Also, it is not possible to link to such images with an cross reference, like any other OurBigBook element with autogenerated temporary IDs.
Another issue to consider is that in paged output formats like PDF, the image could float away from the text that refers to the image, so you basically always want to refer to image by ID, and not just by saying "the following image".
We can also see that such an image does not increment the Figure count:
which renders as:
If the image has any visible metadata such as source or description however, then the caption does show and the Figure count gets incremented:
\Image[Tank_man_standing_in_front_of_some_tanks.jpg]{description=This is the description of my image.}
which renders as:
Figure 12. Source.
Figure 13. This is the description of my image.