I suggest using The GIMP which is free software and allows you to create tilesets quite easily as it supports grids, just as in Adobe Photoshop. I use The GIMP alot more than I use Adobe Photoshop. Mainly due to The GIMP being less demanding of computer's RAM memory, so I reckon it being a time saver just for that reason. It's a great image editing software that can produce and have produced many tilesets for maps in the CS2D mapping community and some of which I have created and some which others have created. It's the ideal tool for map creation.
To obtain this great tool just click the link below, enjoy!
GIMP for Windows
GIMP for MacOS X
GIMP for Unix