This all sounds really easy, it wont be a problem at all to resize and regrid at 100x100 pixel grids. However, I'm still a little unsure about "what" the VTT format is? It's just basically an image that has a consistant size so that the image can be imported and mapped for VTT use? It sounds like my backers might be happy with just access to the high quality .JPGs.

I wouldn't mind going through the extra steps on their behalf to make them a clean version that's ideal for VTT, but the only thing VTT "needs" is an image?