What computer operating system is your Premiere Elements 13 running on? By the way, have you updated 13 to 13.1 yet using an opened projects Help Menu/Update?
I have Premiere Elements 13/13.1 on Windows 7 64 bit and Windows 8.1 64 bit, and I do not find an Image Mask Transition in Expert workspace Transitions or anywhere else. All I find in Expert workspace Transitions/Special Effect are Texturize and Three D.
I do know that there is an Expert workspace Transition/Special Effect/Image Mask along with Texturize and Three D in Premiere Elements 12/12.1.
Where are you seeing that there is one in 13/13.1?