All the images in the strip need to be the same Height AND Width.
This is most important. And not JUST the same size. For example...if you are using png's with transparent backgrounds, each at 256x256. If the images aren't ALL centered the same, you can sometimes get that 'jumpy' effect where your ani seems to jerk to one side from where you had it placed. It may jerk only a few pixels, but it's noticable.
I have no idea what you are animating, but..
Say a face that the eyes are animated and blinking. I make each individual frame and then put them all in one window as layers. Then I
center them
ALL and save each layer , named and numbered accordingly.
If it's a face with steam coming out of the ears, wich changes the size of the image, make your new window (sorry, when I say windows,I am referring to whatever graphics proggie you are using as you make these)..make your new window the size of the largest of the individual ani frames and put all the frames in that and CENTER them.
I can't help with something that is 'alive' and moving across the screen..but if I had to guess...your image size (transparent background) would be that of the area that you want the ani to move in.
Again, I don't know what you use for graphics. I use PSCS and ImageReady. I have found that transferring the psd's to ImageReady to make the ani frames and then saving as png's, for some reason unknown to me, works better than saving each frame as a png and then editing in ImageReady. The latter seems to throw the size and centering off and I get the jumpy stuff again.
Finally, and I apologize if I am telling you what you already know, there is an awesome tool that someone here turned me onto to put the files together and make the ani, called AniUtil. If you don't already have this little wonder, I would be glad to send you one.
I'm also sorry if none of this made sense.
