I made this gif the other day and though I like it, it didn't turn out the way I wanted

http://28.media.tumblr.com/tumblr_lfasaqN7Kn1qakrdzo1_500.gif

see how it flashes white? I used tween too and I didn't want that. I wanted to blend together like this.

http://30.media.tumblr.com/tumblr_lfc8rgWob61qakrdzo1_500.gif

how can I do that without it flashing white?

I use photoshop cs2