how does backpack add an image inplace ?

I'm not totally sure what you're asking for...

It seems like they used AJAX for the adding image section ( where you
choose the file and enter a descriotion ) by putting it in a partial,
and then a DIV that appears when "Upload an Image" is clicked.

The size of the pictures is pre-defined, and thus so is the layout...
they probabl run through @images.each and after three images add in a
<tr></tr> and keep displaying.

If you don't want your users to have to leave the page, do the same
thing... put the "creation" page in it's own Partial, and use AJAX/RJS
files to show/hide the partial's div...

Hope that makes sense...