Try using ":/img/window_large.png". From the documentation for resource files:

Resource Collection Files (.qrc)

The resources associated with an application are specified in a .qrc file, an XML-based file format that lists files on the disk and optionally assigns them a resource name that the application must use to access the resource.

Here's an example .qrc file:

<!DOCTYPE RCC><RCC version="1.0">
<qresource>
<file>images/copy.png</file>
<file>images/cut.png</file>
<file>images/new.png</file>
<file>images/open.png</file>
<file>images/paste.png</file>
<file>images/save.png</file>
</qresource>
</RCC>

The resource files listed in the .qrc file are files that are part of the application's source tree. The specified paths are relative to the directory containing the .qrc file. Note that the listed resource files must be located in the same directory as the .qrc file, or one of its subdirectories.

Resource data can either be compiled into the binary and thus accessed immediately in application code, or a binary resource can be created and at a later point in application code registered with the resource system.

By default, resources are accessible in the application under the same file name as they have in the source tree, with a :/ prefix, or by a URL with a qrc scheme.
For example, the file path :/images/cut.png or the URL qrc:///images/cut.png would give access to the cut.png file, whose location in the application's source tree is images/cut.png.