You need to use ImgBB or Postimage.
I use the latter, where you have to select in advance what size you want your result to be. 340 will give you a small but acceptable one, 640 a nice large one.
Upload your image and a url will apear, on Postimage the 2nd line down is the one you want. You can click on a little Copy button at the end of the line which will copy it to your clipboard.
On the Walking Forum, click on the first menu button on the second line down.
Past the url from ImgBB or Postimage into the box, OK it. and hey presto the photo will appear in the post you are composing.
Hope this makes sense and that I haven't made it sound too complicated!