Are you storing them on a media card? If so the easiest way is to make sure media card support and mass storage mode support are both on (see options/media card on your Blackberry). Then when you connect the BB & Mac using the USB cable your media card will show up on your desktop and in Finder, where you will be able to drag and drop them off of the media card.
If the pictures are on your device memory and not on a media card, the only way I know how to do it is using Bluetooth to wirelessly transfer the file. I am sure there is an easier way (you can probably get the device to show up in Finder somehow), but I don't know what it is.