First you need to upload this to an image uploading site. I use
Imgur because it's much easier to do things like this.
After you upload, you need to right click the image then click Copy Image Url.
Go to the forums, click the Insert Image icon
, then paste the image url.
After that, you got your image
Ok on imgur, you click an image then something like this will appear
You just have to copy that then paste it on your forums. Done.
Whichever way you want to use is your choice.