@mshetzer,
your question deserves a more detailed answer, in addition to what I wrote above.
Let’s say that you have two ways to display your featured images:
1) thumbnail-sized and left floating;
2) medium-sized and centered.
If you want to use a thumbnail size, use these settings:

If you want to use a medium size, use these settings:

So, if you use the first way (thumbnail-sized and left floating), add these lines in your CSS stylesheet or in the Custom Styles panel of the widget:
/* WHEN USING THUMBNAIL-SIZED LEFT-FLOATING IMAGES */
.pis-li {
overflow: hidden;
}
@media screen and (max-width: 400px) {
.pis-li img {
display: block;
float: none;
width: 100%;
}
}
If you use the second way (medium-sized and centered), add these lines in your CSS stylesheet or in the Custom Styles panel of the widget:
/* WHEN USING MEDIUM-SIZED CENTERED IMAGES */
.pis-li {
overflow: hidden;
}
@media screen and (max-width: 420px) {
.pis-li img {
display: block;
float: none;
width: 100%;
}
}
@media screen and (max-width: 979px) {
.pis-li img.aligncenter {
display: inline;
float: left;
}
}
These lines take care of the various mobile devices screen sizes, whether you use a left-floating image or a centered image.
Let me know, please.