Here. I'll show you which one to copy and past into your post.
There's 4 options after you upload a picture. The second one is labled post, and that's the one you want to copy and paste into your signature. So after you upload your desired picture, copy and paste the post part (With the IMG tags) into your signature, and done.
See, do it like that. Upload your picutee you want to have in your signature, and then use the second option on the list it gives you when it's done uploading (The one labled ''post''.) and copy and past that into your signature. I hope this helps.