A guy described the following picture in this way
The picture shows a beach, with many small boats in the water nearby.
That's good, at least to me.
And then he said
Simple summaries like this make it easy to understand the general contents of the picture.
Where he used the word "contents".
Cambridge Dictionary gives some definitions
the articles or parts contained in a magazine or book, with the number of the page they begin on
the amount of a particular substance contained in something
everything that is contained within something
I guess the last one fits with this situation, any object within the picture, the question is
is it natural to refer to boats or any boat in the picture as a content in the picture?