You may have a stroke color (outline) applied around the box.
Select the picture with the black arrow (Selection) tool. Look at the Fill and Stroke boxes (found at the bottom of the Tools, or at the upper left corner of the Swatches panel. The Fill box is the one which is a square, the Stroke box looks like an outline. If there is a color in the Stroke box, it's being applied to the outline of your picture box. Change it to the red slash icon (which means No Stroke).
If that doesn't work, attach a screen capture to give us more information. Use the "camera" icon in the edit window to attach your screen capture.
Sounds like the transparency flattening artifact called Stitching. To prevent it, export to a PDF preset that preserves transparency, like PDF/X-4.