I am in a local photo contest. It has an online application and then provides an image download action. The image download will download images from my microsoft pictures software/files, but doesn't provide an option to download pictures from Lightroom. Because Lightroom doesn't export edited pictures to a source file, I do not know how to download the edited files to the online application.
Assuming you mean upload, not download...
From LR, export your image to a folder on your hard drive.
Using the uploader app, select your exported image.