From one newbie to another, allow me to break it down even more:
1. Save the images on your computer (making specific files for tilesets, sprites, etc. helps.) I recommend naming them something distinctive (like CeliannaA1) because the name that's imported is the name you're stuck with. The files need to be saved in .png format if I'm not mistaken; by default, they will be, but occasionally links will only offer .jpg or something. (Correct me if I'm wrong and I'll edit this out.)
2. Go to Resource Manager. Open Graphics/Tilesets. Import the new files.
3. Go to Database, click on Tilesets. You can create or copy a new one, or modify an old one (bear in mind changing a tileset will alter all maps using it, often nonsensically.) IMPORTANT: What I didn't understand is that the Graphic designations (A1, A2, C, etc.) have innate properties that will affect your ability to use the tiles, so make sure you put the new tiles (i.e., CeliannaA1.png) in their appropriate spot (i.e., A1 (Animation).)
4. Make incredible looking scenery with the amazing art put out by this community and others.