You need to call ImageManager#getImages(<folder>), from the gateway scope.
http://files.inductiveautomation.com/sdk/javadoc/ignition79/795/com/inductiveautomation/ignition/gateway/images/ImageManager.html
Unfortunately, you will 1. have to be in the gateway scope and 2. have access to the shared ‘gateway context’ object.
If you’re in the designer, you can “cheat” a bit, and use DTGatewayInterface.getInstance().getImageList(<component>, <path>) - but this won’t work in the client.