Geoffrey had the problem that every time he uploaded an image by posting a post the image overwrite the previous uploaded one. He asked the Windows Live Writer Developer team and they provided him with a workaround to fix the problem temporary.

Open HKCU\Software\Microsoft\Windows Live\Writer\Weblogs\{blog-id}\UserOptionOverrides\, where {blog-id} is a GUID. You will have several of these, but should be able to tell the right one by looking at the contents of the key.

Add a new String value with name “fileUploadNameFormat” (case matters!!) and the value e
{WindowsLiveWriter}/{PostTitle}/{Randomizer}/{AsciiFileName}

source: Windows Live Writer Image Posting Error