The answers are in the details which appear to be lacking.
The generalized answer is to keyframe the Position property of the Butterfly still image. Is this Butterfly on a transparent background in this particular image?
What version of Premiere Elements are you using and on what computer operating system is it running?
Do you want the Butterfly to track an object in the video or do you just want the Butterfly to float across the whole video or just a portion thereof?
How much do you know about keyframing, changing a property across a clip in contrast to having an effect apply to the entire clip?