Assuming that you already know how to use an image processing software such as GIMP, you need to:
- create an image file which is 512 x 512 pixels for tilesets B to E, or 256 x 512 for tileset A5;
- open your building image in GIMP;
- line up your building image in either of the aforementioned canvas while making sure the images fit within 32 x 32 grids;
- once the image is properly lined up, save the file and import it into your project;
- if you placed your image in a 256 x 512 canvas, then load it in the 'A5' section of your database tileset, if you placed your image in a 512 x 512 canvas, then you can load it in any one of the sections labelled 'B', 'C', 'D', or 'E'.
If you have difficulty trying to place the building images within the grids or canvas, or if you need more clarification, feel free to PM me.
If you provide me with your image file, I can try to assist you in adjusting it to the best of my ability.