Okay, I took a look at the file you sent me in the PM and got the same error. However, the answer is fairly obvious, albeit annoying to solve: your image is just waaaaaaaaaaaaaay too large for it to use. Take a look through it for similar sections (i.e., indoor parts versus outdoor parts and such), and try chopping it down into several different tilesets that are much much smaller. This could take a while given the image size.