There are a few ways to display images on the forum.
Firstly, there's uploading the image as an attachment to the thread. The one restriction is the file size....which can be no more than approx 450kB, a size that is more than adequate for showing something on the forum. Ideally the image should be no wider than 850 pixels. Any wider and the image will be placed in a frame where you can scroll the image to see the missing part. For information, the file size limit is to hold off for as long as possible the point where I need to spend more money on disc space for the site. I have no external funding or sponsorship at present!!
Secondly, you can include an image from an external image host by specifying the url for the image. Again, you need to be sensible as to the size of the image. A maximum width of 850 pixels is recommended, otherewise anyone viewing your post will only see the left hand side of the image....unless they have a desktop wide enough to expand the browser window wide enough to see the rest of the image!
Thirdly, you can link to an external image hosting site....and the image will be viewed on that site.
Personally, I'd prefer people to be sensible about images rather than have to code in restrictions. If I only see half an image because its far too wide then I don't bother trying to work out how to see the rest of it...and the post is wasted!