I don't know how to display that dialog box But I think this is because the browser knows of the image file. If you put something else in the href like say a .xyz file then the browser will show the dialog you are talking of.
Same thing applies to PDF documents when you have a reader in place, the browser opens the file opens it or displays the dialog you are talking of.
Hi, portal framework you are using may stop/override download popup window. If i am not wrong every window is a part of framework once you integrate your application with framework.I have faced the same problem once and i have a provision to specify not to dispaly pop up window in portal framework as below and that's solves the problem. <a href="url?...&pCpslayout=false">payload</a>
You can't force browsers to do anything but, the best way to convice one to prompt the user with a Save/Open dialog box is to set the content type to something that the browser wouldn't have an association for.
When you do this, if you also set the Content-Disposition header, you can suggest that the browser use a particular file name when saving and (if the contentType is known, suggest that the browser not try to open the associated application inline but instead prompt the user with the save/open dialog box.
In our JSP FAQ under JspAndExcel there is an example that sets both the Content-Type and the Content-Disposition headers.