How can I make red and thicker the selection in image picker widget?

First of all thank you very much,

field selected in all forms,

How can I make red and thicker?

(widget visual selection)

Please try to inject the custom CSS code below to the image picker widget:

.divimg.selected {
    border: 5px solid red;
}

Guide: How-to-Inject-CSS-Codes-to-Widgets

Please give it a try and let us know how it goes.