Each block uses two main images.

1/.  Main image (shown in full on hover)
Full colour, jpg, 750x303 or similar ratio.

Use this for:
'Background' div background

2./ 'Icon' style image (used as icon and large icon overlay)
White on transparent, png, square, 300x300

Use this for both:
'Overlay' div background
Image in 'Overlay' Div

Effects:
Set colour overlay colour with transparency as needed.