Added InsertImage.dll and modified readme

URL mentioned to download file was a deadlink and I attempted to build it in MonoDevelop but was not able to achieve any success.  After continuing to do some searching, I was able to find the referenced deadlinked DLL file and install it and get it working.  I've added it to the repo in a release folder and adjusted the readme file with details as to how it works because it's a bit deceptive.

After I installed it I didn't think it was working because I couldn't add an image file or copy paste an image or anything.. But then I noticed the screenshot method.  While adding files would be nice, just having this screenshot method working is good enough for me and far better than nothing.
![screen shot 2014-09-25 at 1 23 31](https://cloud.githubusercontent.com/assets/2615179/4401406/48985012-448d-11e4-9170-b0ff3c534306.png)