hi there,
i had the same prob like you but figured it out.
add the following to the custom css file or directly into the plugin css:
ul.thethe-image-slider-slides li .thethe-image-slider-image img{
border-radius:8px !important;
}
the custom css is missing the “…. img” classifcation. that’s why it didnt work.
tested on mac osx webkit, opera, firefox