You can write media queries in your custom css area.
Basically create 2 alternate image versions 1.) large landscape oriented image that will only display on desktop, hidden from mobile view and 2. a small and vertically oriented one that will display on mobile only and be hidden from desktop view.
Then when writing your css media queries, in order to hide them, target each image with its specific block id; using the "display" property with a value of "none" for when you want to hide it.
Here is an example:
@mobile: ~"only screen and (max-width: 640px)";
@desktop: ~"only screen and (min-width: 900px)";
@media @mobile {#block-yui_replacewithblockidofLargeimage {
display: none;}}
@media @desktop {#block-yui_replacewithblockidofSmallimage {
display: none;}}
Also, you may want/need to add an additional query for tablet sizing. Hope this helps!