change preview image file for this item. pszPreviewFile points to local image file, which must be under 1MB in size
public static bool SetItemPreview(UGCUpdateHandle_t handle, string pszPreviewFile) { InteropHelp.TestIfAvailableGameServer(); using (var pszPreviewFile2 = new InteropHelp.UTF8StringHandle(pszPreviewFile)) { return NativeMethods.ISteamGameServerUGC_SetItemPreview(handle, pszPreviewFile2); } }