How to resize images in org-mode

As of Org 8.0, "Attribute lines now take plists" :

#+attr_html: :width 100px
#+attr_latex: :width 100px
[[~/images/example.jpg]]

As per Jacobo's comment, add the following to your init.el file:

(setq org-image-actual-width nil)

Then in org-mode, you can use this for inline previews of JPGs and PNGs.

#+ATTR_ORG: :width 100
[[~/images/example.svg]]

and if you want to size this for both inline previews and html output:

#+ATTR_HTML: width="100px"
#+ATTR_ORG: :width 100
[[~/images/example.svg]]

#+ATTR_HTML: width="100px"
[[~/images/example.jpg]]

This is a sample on how to resize an image using percentages (Org mode version 9.0.5):

#+CAPTION: Weight space                                                                                                                                     
#+ATTR_HTML: :alt neural network :title Neural network representation :align right                                                                          
#+ATTR_HTML: :width 50% :height 50%                                                                                                                         
https://i.stack.imgur.com/nzHSl.jpg