help to make understandable

I need to write this so people understand.
help make it understandable without example

When clicking on the thumbnail, sometimes you can see part of the picture that isn't in the frame in the thumbnail. When you click on the thumbnail, the picture pops up showing the whole picture.

Here is a picture to help you understand
http://stackoverflow.com/questions/10921048/how-to-resize-pictures-correctly-with-wordpress-gallery-code

regards