In reply to Denni: I suspect that CharlieW has the correct answer. The easy way to check is to pop the card back in to the Mac and then open the Trash from the Dock. If the photos you'd thought you'd deleted are there, all you need to do is to right click the Trash icon and select "Empty Trash", and they'll be permanently removed from the SD card.
OS X treats an SD card like any other data storage device. It creates a hidden Trash folder (called .Trashes) in the root of the SD card and any file that you drag to the Trash icon on the Dock gets moved to that hidden folder. To get rid of the file completely from the SD card you have to empty the Trash.
Windows behaves differently with removable storage devices compared to OS X. In Windows, if you drag a file on an SD card to the Recycle Bin, or select it and click delete in the Explorer toolbar, you get a dialogue box asking if you want to delete the file. If you answer yes then the file is deleted immediately.
Note: Any 16GB SD card
must be SDHC: the original SD standard only went up to 2GB (although some 4GB devices were produced). SDHC is pretty much the expected norm these days, so manufacturers - particularly those at the lower-cost end of the market - often don't bother to mark their cards as SDHC.