First of all, realize that the dongle will only provide a little bit of an obstacle. Someone who knows what they're doing will just remove the call to the dongle and put in a 'true' for whatever result that was called. Everyone will tell you this. But there are roadblocks you can add!
I would find a key portion of your code, something that's difficult or hard to know, something that requires domain knowledge. Then put that knowledge onto the key. One example of this would be shader routines. Shader routines are text files that are sent to a graphics card to achieve particular effects; a very simple brightness/contrast filter would take less than 500 characters to implement, and you can store that in the user space on most dongles. Then you put that information on the key, and only use information from the key in order to show images. That way, if someone tries to just simply remove your dongle, all the images in your program will be blacked out. It would take someone either having a copy of your program, grabbing the text file from the key, and then modifying your program to include that text file, and then knowing that that particular file will be the 'right' way to display images. Particulars of implementation depend on your deployment platform. If you're running a program in WPF, for instance, you might be able to store a directx routine onto your key, and then load that routine from the key and apply the effect to all the images in your app. The cracker then has to be able to intercept that directx routine and apply it properly.
Another possibility is to use the key's random number generation routines to develop UIDs. As soon as someone removes the dongle functionality, all generated UIDs will be zeroed.
The best thing to do, though, is to put a domain specific function onto the dongle (such as the entire UID generation routine). Different manufacturers will have different capabilities in this regard.
How much of a roadblock will these clevernesses get you? Realistically, it depends on the popularity of your program. The more popular your program, the more likely someone will want to crack it, and will devote their time to doing so. In that scenario, you might have a few days if you're particularly good at dongle coding. If your program is not that popular (only a few hundred customers, say), then just the presence of a dongle could be deterrent enough without having to do anything clever.