Before you start replacing your tile sets, i suggest you download GIMP here
. Its free, and once you get the hang of it, you might love it like i do
Then, read questions #14-17 here
. This tells you the size of the tile set templets to use, so you can squeeze as much as you can in it.
Then, open up your project, and go to the resource manager up near the script icon in the upper-right of the window. Click it to open up your resources. Click on the Graphics/System folder in the list. Then scroll down until you see Tile A, B, C, D, or E. (whichever one you want to modify). Select the one you want, and then click Export. Save it to your desktop. Then, while on your desktop, open the picture you just exported with Gimp. Remember you can open multuple pictures at the same time with gimp, so you can switch back and forth between them.
After gimp opens the picture, you want to set up the grid to the size of the tileset your modifying. Up on the menu, click the Image tab, and select Configure grid. Put in the size of the tiles which i think is 32x32 pixles. and select what outline you want to use. I favor the dashed line. After you configure the grid, click the View tab, and check off Show Grid.
Now, to add tiles you need to delete the ones you want replaced. Use the Rectangle Select option and square-off the tiles you do not want, and delete them. Then square off a tile from another Tileset you want, and copy/paste it onto the tileset you want to use. Make sure to move it to a empty tile. Repeat until you have the tileset you want, and save the new picture as a .PNG file on your desktop. Make sure the new picture has the same name as the one your exported. As a backup, rename the one you exported TileA1~1 or something similar, so you can use it again later if you want it back.
After saving the new tileset, you need to Import it into your project. Go in the Graphics/System folder while your project is opend, and click the Import tab above the Export tab. Select the Tileset named Tile* ( * being a1, a2, a3, a4, a5, b c d or e) and click OPEN. Viola! you just imported your new tileset and its ready to use. Make sure to set the Passability correctly for the new tileset. Just click the little Person button in the editor and click each tile to set the passability.
Hope this helps a bit more, and good luck finding new tiles to add and use. You can also use gimp to make character sprite sheets and icons. The Iconset templet can only be so wide, but no limit to how long it is (i think). My icon set has about 2000 icons or so, and i can still add more if i wanted too. When you use other members' things, be sure to give them credit.