Why can't I add a picture into CQ5? Please help!

I am not an IT person and probably will use the incorrect terminology but I am stuck and need someone's help.  I am attempting to change a picture in CQ5 (edit a component).  So I double-clicked the image and the edit component window opened.  I pressed "clear" to remove the previous image and dragged the new image into the "subpage image" box.  When I click "OK", I get the following message:  "Verify the values of the marked field".  I have tried to change/set everything and looked through the internet for direction but did not find any solution to this.  I'm sure it's something simple.  Can anyone on here help me?