Hello,
To alter the mentioned dialog you need to change the code of the file <project location>\CMSModules\Content\Controls\Dialogs\Properties\
HTMLMediaProperties.ascx. There you can insert a checkbox using the following code (this code can be inserted around line 192 where the class textbox is defined (txtImageAdvClass)):
<input type="checkbox" id="chkClass" onchange="var txtClass = jQuery('input[id$=txtImageAdvClass]');
if (this.checked) { txtClass.val('myClass'); }
else { txtClass.val(''); }" />
For changing the label "Tooltip" to "Caption" just change the resource string "dialogs.advanced.tooltip".
Best regards,
Boris Pocatko