How are sprite sheets sized/cut?

How should I size and space my sprite sheet? Every time I try to import my sheet into Construct it cuts it very strangely and pieces of the next frame get included in a previous one. Is there a default rectangle size? The image also isn't centered in the frame that Construct 'guesses'.

I must be doing something wrong...

Thanks!

Nevermind! Figured out that if I have 8 frames and each is spaced 100px apart, it obviously must be an 800px wide image <img src="smileys/smiley9.gif" border="0" align="middle" />

Support for irregular frames would be a great feature addition though!