Map spriting is tedious work, but isn't too difficult once you get the hang of it.
There are plenty of map sprite slots that you can use if you're making a custom hack. If your hack won't have monsters, then you can replace those slots. Moving and standing map sprite slots can be viewed in the FE8 Map Sprite Pointer Table and the FE8 Standing Map Sprite Editor. You can also repoint the tables and add additional slots at the end if necessary.
The Map sprite pointer table refers to moving map sprites. I think (Although I may have them mixed up) that the Animation pointer refers to the graphical data and Another pointer refers to some animation data. With custom map sprites, just toy around with what animation data works by putting various pointers in until it looks nice. That's what I do at least.
The Standing Map Sprite Editor is easier. Just put the pointer to your graphics in and make sure the correct size is selected.
You can stick your graphics wherever with GBAGE, but you have to record pointers. If you use buildfile and CSV, you don't have to record the pointers. You can use labels instead to simplify the process.
There's a field in the FE8 Class Editor with a hex value that refers to the map sprite slots. Just put in the corresponding one there.
Let me know if there are problems or if something doesn't make sense!