Disappearing R markdown image captions

Question

I have an Rmarkdown with an html output.

The following code works well - it places two images side by side perfectly. But is there a reason why the following code does not print the specified captions under the pictures themselves? I have followed the formula exactly - why does the text inside brackets [] not show up as captions?

    ![Caption for the picture.](cropped1.png){#id .class width=50% height=50%}![Caption for the picture.](cropped2.png){#id .class width=50% height=50%}

The following are the document configs:

output:
  html_document:
    theme: "flatly"
    toc: true
    fig_caption: yes
    toc_float:
      collapsed: false
      smooth_scroll: true