If you have ever loaded a map with it's own grid into a virtual table top, you may have come across the issue of trying to align the pre-drawn grid of the map, with the default grid of the VTT itself (this can be particularly annoying if the map has a border of irregular size).
However, with my own maps and tiles it's a pretty straight forward process (or at least it is in Roll20) - so I thought I would discuss the method I use to get the tile to align with the grid (the whole process takes about 10 to 15 seconds).
So, to begin with, I make sure that I have the map layer active...
...and then, assuming the map has already been uploaded to my library, I'll drag it onto the battlemat area.
Now, as you can see, it will rarely transfer at the right size - so right click on the image and choose Advanced > Set Dimensions...
...and because most of my maps are multiples of 12 (i.e. the small tiles are 12 x 12 squares, and the larger tiles are 24 squares), simply change the 'pixels' pull down menu to 'units' and input the number of squares that the map contains (so in this instance I've used my latest tile - which is 24 x 24 squares).
And, as you can see below, that will resize the map and align it perfectly to the inbuilt grid of Roll20.
As I say, the whole process takes a little over 10 seconds - so you should be able to build a nice looking map in just a couple of minutes.
Now, I don't know if it's quite as easy as that in other VTTs (as I say, Roll20 was the first one I became familiar with - and since it works well enough for me, I've stuck with it) - but if there is a similar trick, then feel free to post it in the comments.
Anyway, I hope that was of help. It's the method I use for my own maps, and it seems to work rather well :)
(NOTE: you can find all of my own map tiles HERE ...there's currently over 400 of 'em)