Images won't display

I just started working with Zoho Creater last night and LOVE it! It is exactly the type of thing I've been looking for to share information with an online group. My only trouble so far is that my images are not displaying properly. One image is displaying now, but if I try to load any others it just gives me that little square box with the red dot in it. I also cannot predict this. I initially had three photos display, but as I played around with the program more, deleting and adding files to see it's flexibility, two of those files will no longer display!?!

1) Why is this happening?
2) What can I do about it?
3) Are there image file requirements?

Thank you for this great service!

~zess~