let's say your code looks like this:
(table width=100%)
(tr)(td)
(table width=300)(tr)(td)data(/td)(/tr)(/table)
(/td)
(td)
(table width=*)(tr)(td)(img src='my.gif')(/td)(/tr)(/table)
(/td)(/tr)
(/table)
furthermore let's say you have a screen-width of 640 pixel. so the table in the second collum normally has a width of 340 pixels.
but if you insert a image with a width of 400 pixels here, there is not enough screen-width to display everything on your page without scrollbars.
for an image it's no real problem, because you can set it's width, too. but if you paste a
word in there, you'll get in trouble.