Hi martinicchiola,
It needs code customisation. The code is inside interface (folder) -> functions.php
add_image_size( 'slider-narrow', 1038, 500, true ); // used on Featured Slider on Homepage Header for narrow layout
add_image_size( 'slider-wide', 1440, 500, true ); // used on Featured Slider on Homepage Header for wide layout
which layout are you using? if you are using narrow layout then change the numeric value in narrow layout and if you are using in wide layout then change the numeric value in wide layout. The first numeric value is width and the second is height.
But one thing each and every time you update to our theme you need to change the numeric value you want. So We recommended not to change the code because if you change the code then while updating to our new version all your customisation code will be lost. So better make child theme and first unhook the functions and the edit the code.
http://codex.wordpress.org/Child_Themes
Thank you!
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