If your photos are in iPhoto, you have to export the image so you can find it with the finder.
Since the BB only lets you post 100KB, you have to select an image size and Jpeg quality.
What irritates me is that the choice panel doesn't tell me how big the file is. I have to create it, save it (usually to Desktop for something like this), then look at it -- and usually go thru the whole process again to make it smaller . . .
I can send you an email with screen grabs of my procedure if you want -- but you are probably in Yosemite or Yellowstone or Arcadia or some much more recent OS.
I think all the more literate people on this board post images to some online site and do something cleverer than simply using the board's photo-posting mechanism.