It sounds like you created a new thumbnail image upload section in Add_Entry? Do people upload the thumbnails using the Colorbox overlay window? (the same overlay window used for normal image/file uploads). If so, one thing to make sure
is that your Save button has an OnClientClick property set like how the existing file/image upload buttons are setup. An example one is this.
<asp:Button runat="server" ID="btnUploadFile" Text="<%$Resources:labels,upload %>"
ValidationGroup="fileUpload" CssClass="btn primary"
OnClientClick="colorboxDialogSubmitClicked('fileUpload', 'uploadFilePanel');" />
The colorboxDialogSubmitClicked function is the key here. It moves the upload control and button (your entire dialog) back into the <form> tag which is needed in order for a submit button to work.