Featured Image

Trying to change or add a feature image to my events in event calendar but for some reason it won’t let me make the change. If I try to change a feature image for an event I already have it just closes import image window without doing the change and when I try to add a new event and add an image from the library it just again closes the window without adding or changing a thing. Can anybody help me out with this problem please. Thanks for your help!