hmm well when I tried (codeword here is "tried") to make/add a tileset in-game, the first thing I did was try to fit it into a grid template, (a checkerboard of sorts)
basically I made sure each element on the tileset would fit into one square on the grid and resized/reduced manually any errors while fitting them
(I did this with paint, so sue me XD I still use it since I haven't learned to use other painting programs XD)
here's a tileset grid sample you can use to resize your current tileset into RPGXP, as a tip make sure it's saved as .png to avoid blurs and make sure
to have all the unnecessary background you want to remove as a single color.
tileset grid:
make sure to save as .png
wish I could help you with the tileset editing itself but I've simply got my hands full atm, sorry.