You could also try to change the x and y setting in the bump section scale to a number larger than the z. That should grow the bumps so they stand out from the tree more.
Try this. Get rid of the bark picture and just set the tree to a white color. From there adjust your bumps till you get the effect you want them import the tree bark picture back in. It's hard to see what's going on trying to look threw the image of the bark. I have a bark here that just has y adjusted to 2 and x and z one. Also you may want to abandon the bump image all together and make your own. You can get big bumps by changing the filter in the bump section to something more coarse. It may be harder to make it look real though. I just made one with the wood filter and heavy terrain funtions and it looks pretty bumpy.