Images not visible in epubs in Ibook

Hi.

I try to create an epub containin lots of images. All the images are of maximum size 500*500.

When trying to view the epub in ipad, a few of the images are not seen. Only their captions are visible.

But when I double tap the place where the images should be actually present, the images open fine in full screen.

At times, the imae just appears for one second and then vanishes.

Has anybody faced similar issues? All imaes I use are jpg files.

Any help in this regard will be greatly helpful.