Piskel and other editor are great to make graphics... but then, you will need to copy/paste those graphics in NESmaker's tilesets. Because the the resolution of your images might be the ones NESmaker accept.
For example, look at this tutuorial about the Pixel Editor in NESmaker (you can open an image, select copy... then open a tileset and paste your graphics inside it):
https://www.youtube.com/watch?v=xrvJcQqkopg
And there are more videos about Pixel Editor and graphics... on the Youytube channel (here's another older one:
https://www.youtube.com/watch?v=tV9TF9T7Avk)