I have recently upgraded from 2.4 and since then have lost the ability to select images using the TreeDropdownField

when I click on it, nothing happens.

example

$fields->addFieldToTab('Root.Content.Slides', new LiteralField ($name = "literalfield", $content = "<h2>Left Slide Window</h2>"));
$fields->addFieldToTab('Root.Content.Slides', new LiteralField ($name = "literalfield", $content = "<h4>Slide 1</h4>"));
$fields->addFieldToTab('Root.Content.Slides', new TextField('SlideDescriptionShort1', 'Short description'));
$fields->addFieldToTab('Root.Content.Slides', new TextField('SlideDescriptionLong1', 'Long description'));
$fields->addFieldToTab('Root.Content.Slides', new TreeDropdownField('SlidePage1ID', 'Page', 'SiteTree'));
$fields->addFieldToTab("Root.Content.Slides", new SimpleImageField('SlideImage1', 'Image'));

I may be confused by your questions, is the TreeDropdown notworking or the SimpleImageField? You say you can't slect images with TreeDropdown but your example calls a apge realtion.

If you're refering to the SimpleImageField,its been depricated and you should use UploadField http://api.silverstripe.org/3.0/forms/fields-files/SimpleImageField.html

One of the great things with SS3 is Images can be a $has_many relation, and there is Gallery module that has a sortable UploadField for images that works quite well, so it may be worth looking into upgrading your code there too.

Image link
http://imm.io/S6dX