SIdebar (text widget) is shrinking?

Hi,
I’ve pasted an image as a header in my text-widget in my sidebar. It’s 310 px. But somehow the widget seems to shrink it to 283 px. I’ve defined my sidebar as

div.sidebar {
float: left;
width:310px;
margin-left:50px;
}

I’ve also tried to make a .textwidget with the width to 310 px.

I want a really simple layout. 50 px margin left for the sidebar, which should be 310px, the 70px gap to the content wich should be 600 px.

Any ideas?
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