I haven't used the AniUtil, but I'm pretty sure it just creates a single image of the frames from left to right. This is the kind of image that DX uses foranimations anyway.
So, what you have is actually a loong image. And when you set the frames in the Animation panel, you are telling DX exactly how many times to chop up that image. Since you have the mouse over set to one frame, DX assumes you just want to show that loong image, it doesn't know from "frames".
Which is why I don't think you'll be able to get the effect you are looking for in this manner. If I understand correctly, you want an animated image (in this case, something appearing to rotate) which will stop WHERE IT IS when you mouse over. And "where it is" is actually just showing one fiftieth of a single image. I am not aware of anyway to know what frame (or arbitrary slice of an image) an event (such as mouse-over) happens on, even with scripting.
A diffeent approach, again if I understand the question correctly, would be to write a short script that would ACTUALLY rotate a single image (not the long image, but like the first frame only of it). You could easily stop this rotation with mouse over with your script.
DX supposedly supports scripted animations as well, but in my experimentation I have never had much luck with those commands, in particular stopping the animation in the middle.
But, you probably didn't want to script anyway. Normally I would say use the same long image for both mouse over and mouse away, giving both states 50 frames. For the away state, check the loop box and the INTERRUPTABLE box, and the "Style" (forwards, backwards, etc) of your choice. For the over state, do NOT check loop, and use the Forwards or Backwards style, this will run the animation once through when you mouse over, stopping at whichever end you picked.
Whew, getting long winded here. Anyway, because you're using a 50 frame animation you may have the undersired effect of having to show an entire long animation when you mouse over. So, you can instead use a completely different image file for this state! In this case, one of the animation frames saved by itself.
ok then. got to go, hope that helps, not too confusing. Experimenting is best way!