Sorry, I misread it as just an image (as in <img> tag) and not an input element of type image.

For some reason, I can't seem to access the specific element when its type image via the forms item collection. However this code seems to work:

Code:
        Dim MyImageElement As mshtml.HTMLInputElement = DirectCast(GetCurrentWebDoc.all.item("ctl00_Main_postComment_postcommentImageButton"), mshtml.HTMLInputElement)
        MyImageElement.click()