In your case there are two ways to remove the black bars. None of them may be 100% perfect.
The first one it to make your timeline as big as the still image, or vice versa depending of the display ratio of the image. The other way is to accept the resolution of the timeline and scale up the image so it fills the video frame. This will of course lead to that parts of the image will be cut off.
Set to frame size makes sure that the image fits into the video frame without leaving parts of the image out. It all comes down to display aspect ratio. (is display aspect ratio the correct term here?)
But, if you have let´s say 1080 video that is 16:9 and mix it with images that not are 16:9 you need to accept the black bars left and right of the image. Thats all i can say without more details about your Sequence Settings, video resolution, still image resolution.