Lord, I think in general terms I disagree but in this case the tiles have a lot of random detail so you would get away with it. I did a tutorial discussion about the tech side of images which covered 8bpp and stuff so for my views on that I think that thread goes into more detail on why it can be good and where its not so good.

GP, I think 300dpi is top end. There really isn't much need to go higher. From a high dpi you can resample lower so its always nice to have too much. 200dpi should be adequate esp for printing and viewing at 1:1 scale. I would say that 100 is not enough for printing but you could get away with it for a VTT. But 50 seems to me absurdly low.

The file load time is odd in this case. I thought that the idea was that everybody would have all the tiles and that its the use and arrangement of them that makes the map. Wouldn't it be easier to get all of the players all of the tiles and then make up the map from using these images with no image download ?