You have a still image of 'something' against a white background and want to superimpose it in a video right?
A couple of ways of doing this.
Probably the best way is to open the image in a decent image editor, remove the white background and save it in a format that will preserve the transparency. I'm an Adobe fan so I would save the image as a Photoshop PSD file. Import that into Premiere. Drop it in a sequence as layer two. Job done.
If editting the image is not an option (and I can do it for you if you can't as this is easy enough - aren't I nice?) you can add it to your sequence as layer 2 and drop a 'Luma key' on it form your video effects folder. It may need a bit of masking and playing to get right but it should work ok unless you have lots of other pure white in the image.