So this is similar to the issue I have faced for a while in yEd. I create network diagrams with images of the equipment used. When you connect the ports the line only shows to the edge of the image.
I just found a way to get around this with a little trial and error. For this, I used paint.net, in the image use the magic wand to select a section or create a square inside the image then press Delete on your keyboard. This will make the selection area transparent. Save the image as a PNG as JPEG doesn't support transparent sections.
In yEd now drag and drop your new image. Join this with a line to another item, click the line to show the end of the line in the centre of the image. Move this into the transparent section and drop. You will now have a line shown over the image to the new transaction section.