nivo slider in a widget

I’ve read you have to use a text widget to put the nivo slider in a widget, using the [nivo] shortcode. But what I still don’t know is how to include the images to this slider, as it’s no a page or a post with media associated. I’ve read the syntax includes some options, like post_type=”attachment|post_type – Post type to display” but I don’t know how to use them. For instance, what value should I have to give to post_type (post_type=”I don’t know which string goes here”) to make the images I want available to the slider. Or If I have to use another option instead of post_type.
Thanks.

https://www.ads-software.com/extend/plugins/oik-nivo-slider/