Adding #+attr_html: :width 100px
above aa image link changes the width of the image in the exported html file. See this post for more info.
However, I wish I can set all images' widths to 100px
in the exported html file without specifying #+attr_html: :width 100px
for each image -- it's too much work. For some exceptions, I wish if #+attr_html: :width 200px
is added, then the method should respect it.
If all I care is about the widths of images within org-mode, then this post suffices by adding (setq org-image-actual-width '(400))
to the config file. However, that does not take care of the images widths in the exported html file.