- Put some guides on the image using Image*Guides (if there is no Guides on Image menu, you need to install gimp-python). Tips: you can use Rectangle Select first, then Guides*New Guides from Selection, add more as needed. When using Guides*New Guide, move your mouse to find out the coordinate on the bottom-left corner.
- Filters*Web*Slice (again, if it’s not there, install gimp-python).
- The created HTML is just snippet, not complete HTML. Use it as your starting point.
You can read the explanation of each option on Slice dialog box here.