CSS info window

Hi. I am trying to alter the way the info window looks when you click on one of the placement markers. I have carried out the instrutions for putting the CSS into a child theme but in any case even if I alter the main child theme CSS it makes no difference.

The window pulls in a picture from the post. The parameters of this are:

img.attachment-post-thumbnail wp-post-image

If I set this into CSS:

img.attachment-post-thumbnail.wp-post-image {
max width:150px;
}

No change. Would any appreciate any help – maybe I’m just missing something in my code?

Thanks in advance.

https://ww.wp.xz.cn/plugins/geo-mashup/