Unfortunately there is currently no easy way to do this. I am planning on an image browser function where you'll be able to remove images selectively. That is planned for a future update but not yet developed. There are two ways you can do this now:
1. Back up all your databases and images to the desktop using the conduit or Desktop Connect. Remove the app from your device and then redownload it from the App Store. You won't be charged again if you're logged in as the same iTunes account user. Then install the databases and the image files you want.
2. There is a desktop app called PhoneDisk that I've used to remove files independently like this. Here's the link:http://www.macroplant.com/phonedisk/
You mount your device and can browse the folders. HanDBase images are stored in the Documents/Images folder under our app. This is the easiest way to do things right now but does require some external software. Please be careful not to delete anything you do need if you use this method!
Hope this helps and sorry there is no easier way yet available.