During page load, all child controls receive the OnLoad event. When you try to access controls defined in the control's markup in the OnLoad method override, all markup controls are NULL.
To solve this mysterious problem, you must register the custom control's tag using its relative URL path like:
<%@ Register Src="~/CustomControls/MediaGalleryPostThumbnail.ascx"
TagName="MediaGalleryPostThumbnail" TagPrefix="CC" %>
MediaGalleryPostThumbnail.ascx is the implemented custom control here.
