Starting a new tutorial series on some photoshop tips for working with and converting battlemaps in Photoshop. Most of these techniques will also work in tools like GIMP, but with slightly different instructions.
Start with:
Top Down 100 pixel square battle map
Creates
Isometric map for 74 px hex or 64 px isometric grid
1. Create a new Image
2. “Place Linked...” your battle map
3. Put the map layer in a layer folder
4. Transform The folder
78.4% height and width
45 degree rotation
5. Transform The folder again
57.7% height only