Update - 17 July, 2012
In Thesis 1.8.4 the concept of 'Design Mode' was introduced.
If you make design or layout changes but they don't show up when you check the front of your site, you're probably not using Design Mode.
Original article starts here:
One of my customers had a header and footer specially designed for their Thesis site recently.
And they wanted to know how to change the settings in Thesis so it displaid the new image properly, given that it's a different size.
This is a pretty simple process that involves going to your Thesis Design Options screen and re-setting the number of columns and/or the width of the columns.
Here's the process
Access your Thesis Design Options screen:
In the Site Layout section, click the '+' sign beside 'Columns'.
Either select a different number of columns from the drop down menu (1 below) or adjust the width of the existing columns (2 below), or do both as necessary:
Start by making sure the total of the pixel-widths is the same as the width of the image you're installing.
However, keep in mind that there will be padding in the content and sidebar divs.
If you know how much the padding is you'll need to reduce your pixel widths by the same amount.
If you don't know how much the padding is, you'll find that your image still doesn't fit, even though you made the total of the pixel-widths the same as the image width.
So take a look at your site and see how it looks.
Then keep adjusting the widths of either the content column or your sidebars until the image fits properly.
Once your image is fitting perfectly hit the Big Ass Save button
That's it - you're all done.