As of this writing, Maxis has been rumored to be working on a utility for converting SCURK tilesets to work with different platforms. It remains to be seen if and when this utility will come out, so many will have to resort to the outline below.
For now, the only current path for converting raw SCURK tilesets between platforms seems to be from DOS to Windows. Thus, the only way of distributing SCURK graphics between the remaining platforms is to save tilesets in image format. The most universal and reliable format seems to be GIF. You may notice that many sites store their graphics as simple GIF images that are displayed right on the page, allowing you to simply use your browser to save them to disk. However, that's where things get a bit tricky.
At this point, you'll need to convert the GIF image to a format that you can use for importing into SCURK. For DOS, you'll need to convert it to PCX image format, using any number of Windows or DOS paint programs that support multiple image formats. For Windows and Mac, virtually any image format is usable, because you'll be able to simply use your operating system's paste and copy commands. In doing this, you only need to load the GIF into a paint program, and then copy it to the clipboard. From here, you need to paste the image into an empty SCURK tilebase, and erase the background. If you are without a decent paint or image editing program, check out www.shareware.com and select to do a search for "paint" and/or "GIF". You can customize the search to look for files specific to your platform.
The easiest way to erase the background in SCURK is to select the transparent color from the palette and then use the fill tool to fill in the background. At this point, you can simply save the resulting tileset and use it with SimCity 2000. This is actually more flexible than using tilesets because you have the control of putting each building in the tilebase of your choice.