Featured Grid Image Size

Hi there. I really like this theme (and will likely upgrade to Pro) but I'm having a problem with my image grid. Two of my grid images have grey borders around the bottom and right edges, as though the images are not large enough; these borders change size (and sometimes disappear) as the responsive screen shifts when it is resized. Can't include my site address, site not yet launched. I have tried using all of the suggested sizes from the Theme Instructions (780 x 439 for featured image, 800 x 450 for featured grid, etc.) without success. I also used a pixel ruler on my internet browser to measure the size of the grid tile with my screen maximized (800 x 530 pixels) and resized the image to that size; also did not work. What sizes (pixels) should my images be so that they fit the grid at any responsive screen size? This theme is gorgeous, but it looks kind of ghetto when the grid doesn't work. If I can get this fixed, I'll definitely buy the Pro version. Thanks!