Gosh, what a thread ! So much information and so much misunderstanding.
The other day on some board I saw that some chap ask for a high res board game and the map was produced here by somebody. He then said that this map was 7200 x 4800 pixels and was for a board 24x18 inches and at 600dpi. Now my math is not stellar but even I can see something is amiss there.
(Edit -- Ahh found it... http://ritz21.deviantart.com/art/Myt...d-Map-70277509 got the numbers a little wrong but comment still applies ! Well I guess you can print it at 600dpi but it wont be 600dpi)
One thing that has been overlooked is that most image formats embed the DPI into the file spec. A png file has its DPI written into the format as does BMP. I am fairly sure TIFF does but I dont know about GIF or JPG. Most paint packages allow you to scale by DPI as well as image size. You can scale in PSP by a different DPI and keep the image the same res with its Actual Size box.
A post earlier was suggesting that it does not make any difference with a VTT what the DPI is. I believe that *in general* this is incorrect as some of them scale the map based on its pixel res. I.e if you want something 5feet across then it had better be exactly 200 pixels if you want all the icons on top to be in scale which makes large maps very difficult in those VTTs.
Since I write ViewingDale I can add a little more info too about VTTs. For some of them - namely mine - the res does not fundamentally matter as it is a zoom browser where you can enter any image for any scale tho whether its of high enough res to make it usable is another matter. So for me 200 pixels per 5ft scale may not be enough in certain circumstances - for example if you wanted to print that map for miniatures at 1:60 real scale on a commercial print job. So I would welcome the large gigapixel images for poster sized maps - bring it on !
ViewingDale maps can be export at any DPI up to about 7200. The scale of that image is independent of the DPI but the real size, DPI and map scale are interrelated so that you can ask for an image which is 200 pixels for 5ft if you so wished.
Finally, I would suggest that if you do want to use a map in a VTT then its better not to have a grid on it as all VTTs that I know of can put one on at the selected scale. DM's also like for it not to have fixed items like monsters and stuff like that where icons will be used on top and moved dynamically. Now how dynamic something has to be before people like it is very variable.
As far as I can see, an image is just a bunch of pixels. If that image is supposed to exist in the real world at a certain size then it has a PPI / DPI value for that real world size. That value might be very different on screen or when scaled to print via a printer driver.
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