Edit your theme > CSS > core > global > plugins > tthumb.css
Find
.bim_tthumb_wrap .tthumbimg, .tthumbPreview
{
border-radius: 4px;
/*-webkit-box-shadow: 2px 2px 5px 0px rgba(224,224,224,1);
-moz-box-shadow: 2px 2px 5px 0px rgba(224,224,224,1);
box-shadow: 2px 2px 5px 0px rgba(224,224,224,1);*/
}
Replace by
.bim_tthumb_wrap .tthumbimg, .tthumbPreview
{
border-radius: 50%;
/*-webkit-box-shadow: 2px 2px 5px 0px rgba(224,224,224,1);
-moz-box-shadow: 2px 2px 5px 0px rgba(224,224,224,1);
box-shadow: 2px 2px 5px 0px rgba(224,224,224,1);*/
}
Save
You should upload thumbnail manually for the best display.