Dear pbwbart,
You can open file "CuteSoft_Client\CuteEditor\Dialogs\InsertImage.aspx", you will find the following line in this file:
<div id="divpreview">
<img id="img_demo" alt="" src="../Load.ashx?type=image&file=1x1.gif" />
</div>
If you remove <img id="img_demo" alt="" src="../Load.ashx?type=image&file=1x1.gif" />, run cuteeditor, and click button "Insert Image", you will find image file cannot be previewed.
You can replace "\CuteSoft_Client\CuteEditor\Images\1x1.gif" with your own large size image, don't change the file name, If you click button "Insert Image", you will see your own image file will be displayed at the right preview panel.
Thank you for asking