Resize the featured images for the Services Displayed


I have installed (Great Theme), but I can't seem to use the Service-Mode - when clicking on the Add Service button nothing happens...

I've decided to go with post-mode which works great, but my featured images are not resizing to fill the full width of the Service blocks created on the front page.

My featured images are sized 420w x 280h to make use of the Image-Slider - (I thought that it would be too big?)

What can I do to still make use of the image slider and have the images fit to the service blocks?