Small Useful Things About the GM Image Editor

SUTAGMIE, yes. 
1. The transparency colour of a sprite is determined by the BOTTOM LEFT corner pixel in each image. If in that corner it is the colour of, say, lime green, then all of that colour in the sprite will be transparent.
2. You only get ONE undo, so if you are really going to try something, copy and paste it into paint, or have a backup copy.
3. The origin of a sprite will determine how it is placed when used as an object, the origin coordinates can be changed in the first window, the one that appears before you begin to edit your sprite. 

I will be adding more in time.

Fixed.