The harder way: Create an album, upload media (images) to that album, set the permissions on the album and then link the media in the album in your reply. With that method, what people see is dependent on how the permissions are set for that album (ex: everyone can see, only members can see, only specific people, etc.). The image above was created using this method. An album was made with one image in it (the instructions) and then the permissions were set so that everyone can see it.
This forum is currently configured to limit the size of images to 4096 KB (about four megabytes) and 1200 by 1200 pixels. This is more than can be displayed on most computer screens. You can use any of the common imaging tools to reduce the size of your pictures or a free online image size reducer like the one at https://www.reduceimages.com for example.