I'm trying to mask an image inside the area of an imported image. However, it masks the image inside the imported image's borders instead. I have no clue how to make Inkscape realize that I want to use the imported image's space inside the borders as the mask, or how to fix the imported image itself so that Inkscape stops using the borders as the mask. I also tried clipping but it yielded just blank space. Here is the process of what I went through:

Make the image invisible for a moment and take the Fill bucket tool with some "over-filling" setting and Fill the inside of the frame. Select this frame with the visible image and go Object->Clip->set clip.

Example