A typical workflow would be: Import from card/camera to Lightroom, edit/adjust images, select images to Publish by adding them to the SmugMug Publish area, then upload by clicking Publish. Exporting is saving another copy of an image for some purpose (to a harddrive, or perhaps in a different format). Publishing does that and more. By Publishing, you don't have to store a new local copy, and any conversion is done behind the scenes based on your Publish settings, then sent to SmugMug in one easy step. Better yet, any future edits you do to those photos in Lightroom will optionally mark them for re-publishing on SmugMug. This includes replacing images in customer orders via Proof Delay. Re-crop, remove red-eye, blemishes, adjust exposure and noise, then click Publish again and the image is automatically replaced in customer orders still in Proof Delay.
SmugMug's organization is Category>Subcategory(optional)>Gallery, e.g., Animals>Elephants>African Greys It's not necessary to create a new gallery for each upload, but you can.
More info on Proof Delay and SmugMug's LR Plugin.