Wrong forum - this is really a question about using PSP9, not AGS. Which Is why I've split/moved it.
The simplest way to make a mask is to add a new raster layer (either from the 'Layers' menu, or by pasting something in as a new layer - Ctrl-L in PSP8) and draw the walkable areas on that. Or, use the line tool to draw the areas on a vector layer, then convert it into a raster layer. Then fill everything that isn't a walkable area with black. Decrease the colour depth to 256 ('Image' -> 'Decrease Colour Depth', or Ctrl-Shift-3 in PSP8). Don't worry too much about the colours matching the AGS palette - it loads the areas from the first 16 palette slots, regardless of their colour.
Then, when you're done, use 'Canvas Size' to add the extra pixels to the bottom, save it & it should work fine.