I've cobbled together an extension *AutoThumbBox.cs* that will both Convert your posted images to pre-defined thumbnail sizes and add the code for lightbox support at the same time. Just add this extension, set up your thumbnail parameters and you are good to go. Whenever you post an image, include atb="[YOUR FLAG]" in the image tag. The extension takes it from there. AutoThumbBox also handles max dimensions, to auto size your thumbnails based on a max width, max height or both. You can store multiple thumbnail sizes for your different needs. Here it is:
AutoThumbBox.cs (31.31 kb)
Check it out, and put it to use!