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@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.