There is some programs to create the spritesheets, like texture packer, which is not free. You can search alternatives in google or tutorials on youtube
TexturePacker Alternatives
ShoeBox. Free Adobe Air app for Windows and Mac OSX with drag and drop - and clipbord Game Development tools. ...
Free Texture Packer. ...
Sprite Sheet Packer. ...
rucksack. ...
Alfred Spritesheet Unpacker. ...
Sprite Sheet Animator. ...
Zwoptex. ...
GameDevUtils.com
What I did is create a template with a common reference point (in this case the head) and manually add the sprites by changing the transparency of the layer. Then you remove the transparency and the template, and export the png. Works for me, but im not sure to recommend this.
