Hotspot splash page not always showing .png graphics file

I have a MT hotspot (v2.9.30) running on RB532. Around half the time I open the splash page, the graphics (i.e. the .png file) is not shown (just an empty box with a little red cross in the left top corner), but text and other formatting (login box, etc) appears correctly. The rest of the time the page loads correctly, showing the .png file. I have not edited the standard splash page that comes with the router.

Ideas anyone?