[RMVX ACE] HOW TO PROPERLY ISOLATE AN IMAGE FROM TILESET.

This may not be the place for this. If not, I apologize.
This is what I did:
I exported a tileset to my pc. Then, I took a screenshot of it and opened it in Paint. I cropped a bookcase and made sure the final size was 32x64. I followed a tutorial for Gimp and made a 3x4 block, put the image in all 32x64 blocks and exported it as png to my pc. I changed the file name to add the $. Then I imported it to my Resource Characters. I made an event with it and move away from player and it worked perfectly. The problem is that it is a different size than the original which means my cropping was not accurate. So the question is how do you isolate an image from a tileset and be assured that it is the proper size?
Thanks