Adding Thumbnails to Posts in Certain Categories?

OK, so been at it 3 hrs last night, read multiple forums and sites, tried the “add featured image”, Tried “Custom Fields” – using “image” or whatever then image link in wp, then adding code in the loop to match – still not having it.

Any idea’s would be magnificent!!!Please!

Here’s the link to how it currently looks, Here

Should I make the images the same size?

Thanks if anyone can advise.