images in treeview cells are not activatable by default. It can be done, but one has to do some effort for it, like a custom cellrenderer class derived from CellRendererPixbuf.
There are examples for it on the web, likehttp://stackoverflow.com/questions/4940 ... ns-on-rows
orhttp://stackoverflow.com/questions/2821 ... llrenderer
for Python, I've seen an example in C before, but can't find it in the moment.
For a solution without an image, a GtkCellRendererSpin would be much easier, but if has to be an activatable image, I don't know of another solution than the ones suggested.
Regarding effecting the states of cells in descendents that shouldn't be a problem.