How to use/access an image file from an upload form using javascript?

I am currently editing my install of elgg to trigger a process in Javascript once blogs are posted. The idea is to crosspost the blog to other social networks using Javascript code. This works, except that I am using blog_tools to allow users to upload an image to their blogs and when I route the upload form's data fields to the blog/save action via javascript, the image doesn't make it through. I can see that the image is saved in PHP using saveIconFromUploadedFile() - but I'm not clear on how the file could be accessed / uploaded in javascript. It seems that I need to trigger the crossposting code after elgg has finished saving the blog and has generated a thumbnail url for the image, but this presents another problem. I could use a hook in PHP to trigger code once the new blog has been saved, but I would have no way of launching the javascript at that point that is needed to be run to crosspost to the other network!

I suppose maybe I could forward to a new custom page after blogs are created that process the crossposting in javascript - but that seems clunky. Anyone got a more elegant solution?
Thanks