I'm no expert unfortunately but i managed to work out if i uploaded my photos to my you tube channel as a slideshow i could transfer pictures from my computer and store them. When i need to use one i right click and select copy location. i put the cursor back on the blog i am writing and i go to the menu and select the insert picture tool/button and paste, then i select the tick at the end of the box and my picture appears where the cursor was. The first picture posted is the one that is selected for the thumbnail i think. That is all i do but there are dedicated sites for doing this possibly called steemed.com, not sure. hope that helps