Yes, I mean Telerik.Web.UI.dll. If it is bigger, there could be problems with some shared hostings, as the DLL loads in the memory of the web-server, and there might be some limitations for the usage of the RAM. Also bigger library slows down the loading of the application.
In most cases we are using gif images in our controls, and in some of the cases we are using both gif and png images using hacks for IE6. This doubles the size of the used images and increases the CSS code also. Basically, if we would had png and gif version for every image, the size of the DLL would be at least 50% larger.
It is common situation in an application to be used more than one skin for one and the same control. Especially with the Window dialogs where you could use different skins to express different meanings.
I agree that when building an application is boring to override the base resources. But at least if you have one external CSS linked to your application and such a fix will take no more than 5 minutes with one additional image and 3 lines of CSS code.
That case particularly is reported first time, and we will do our best to put some extra images in the common shared resources in order to fix the problem with the black skin in the Window dialogs for the next official release, which is scheduled for the mid-March, 2011.
the Telerik team