You need to use a website like imgur to host the image. Make an account, don’t worry about making albums or anything, you can go straight to uploading. There’s a button on the main page for uploading from your computer, upload pics using that.
If it doesn’t automatically bring you to a list of unsorted, uploaded pics of yours, you can click the images tab in the upper righthand corner. Click on your pic and it will give you a list of links.
You want the one that looks like (img)http://i.imgur.com/5wH8YQa.png(/img) but the ‘img’ bit will be in square brackets. Copy that text and paste it into your forum post here and it’ll show up.
(protip: you can hit the insert image button here in the forum’s post options, and paste any website image’s url in there. Same idea.)