Images not showing immediately in published file

My eLearning has a fron page with images as the table of contents. Each image has a clickable hotspot over the top to navigate through the sections. When previeiwing the file it looks fine but when I launch the published file the images aren't showing. I can see the text but not the images. When I click on a hotspot to move to another page and then go back, everything is displayed properly. 

When I rollover the images the rollover text is displayed, everything is working as it should, you just can't see any images.

Even the background images on the Master slide aren't showing until you navigate back to the page.

All images are set up to appear after 0 seconds and for the rest of the slide. The pauses that have been added are all set at 1.5 seconds. So why aren't the images appearing until you navigate back to the page? What am I missing?

Any help would be very much appreciated.