When the Editor is set with Skin=Window7, fieldsets has a non designed margin top value. The New Area bottom also drop to a new line, where it should be next to the radio buttons. 

Possible solutions is setting a CSS file to the DialogsCssFile property with CSS rules that resolve the issue:

<telerik:RadEditor ID="RadEditor1" runat="server" Skin="Windows7" DialogsCssFile="~/Styles/Styles.css">
</telerik:RadEditor>


Styles.css

html.redImageMapDialog.RadForm_Windows7 #dialogControl fieldset {
	margin-top:0;
}


button#ImageMap_NewArea {
	width:110px !important;
}