You have probably entered contradictory settings for the table and cell/column widths. When this happens Help+Manual automatically sets the widths of all columns to dynamic and they then adjust themselves on the basis of their content. You can usually correct this by setting at least one column with a dynamic width so that you can "lock" the width of the other columns.
You can turn column width locking on and off with the Lock Column tool in the Table tab. The tool is highlighted when the cursor is in a column that is locked.
When you place graphics on the page without tables they are automatically resized to fit on the page in PDF. This is not possible for graphics in tables, however, because here the table cell and not the page is the container for the graphic. When you insert graphics in tables you need to make sure that your layout will fit on the page. In addition to graphics clipping oversized graphics in tables will also switch all column widths to variable in PDF, which will probably mess up your layout.