hello, i uploaded two images both 1920 x 550, then i set my theme options to display slider with two slides, next i created two posts added dummy text and set the featured images to the two files i had uploaded, when i run view the page the 1920 images are reduced to 75px thumbnails, i have been messing around with this all night and i cant work out whats happening 
my website is at briangoodall.co.uk/jed
Since there is no 75px by 75px images defined by theme it must be that some plugin does generate those small thumbnails and forces slider to use them. Try to disable all plugins and then upload another image just for testing purpose.
Let me know if this helps.