This is one of Arachnophilia's more interesting features.  It will automatically calculate the height and width tags for graphics that you insert into your documents.

My thoughts on this feature are mixed.  On one hand it is helpful, on the other it is a hindrance.  But I have to make a decision, so here it is:  turn this feature off

I'll explain why in a moment, but first here's how to turn it off:  Tools | Options | Miscellaneous tab, remove the checkmark beside "Calculate and Update Graphics size tags" then hit Okay.

So, why did I have you turn that off.  First of all, it doesn't work all the time to begin with.Graphics with multiple part names confuse it.  That is, an image named "image.gif" will have the size automatically calculated.  An image named "image 2.gif" will not.  The space in the file name confuses the feature.

Also, there are a couple of places in my toolbars where I have pre-set the height and width of an image because we don't want it to display until it is needed (the Table Background script leaps to mind).  If the Auto-Calculate option is turned on, it will destroy the height and width tags that I set.

Thus, the best option is to have this turned off.  However, under certain conditions, there is one situation where this feature comes in extremely handy.  Have you ever had trouble knowing where to set a margin?  Usually, it will take you two or three tries to set a margin by trial and error, however if you use the Auto-Calculate feature, it can tell you how large your background image is so you will know exactly where to set the margin.

To do this, once you have your stationery nearly completed and are just about to set the margin, position the cursor on a blank line in the code then use the Image button on the Structural Toolbar.   When prompted, pick the same image that you chose for your background.  When the code appears in the window, click to place the cursor within the filename portion of the tag.  Now, on the tools menu, pick "Calculate Graphics size".

Even though the Auto-Calculate option is turned off, you can still use it manually.   If the filename is a single word as I described above, then the height and width of the graphic will be filled in.  By adding about 10 or so to the appropriate dimension of the image you can see what value to assign to your margin.  Just remember to delete the image again (pressing undo immediately after you discover the size is an easy way to remove this extraneous image).

For Example:  I'm creating stationery and I wish to know where to set my margin.   Using the Image button on the structural toolbar, I insert the same graphic that I used for my background image.  I position the cursor in the appropriate spot then I go to Tools | Calculate graphics size.

Arach will do it's thing and if the filename is in the supported format, the height and width attributes will be filled in.  Since I am creating a Left Border stationery, I need to know how wide the image is.  By adding 10 or so to the width of the image I have the value that I should enter into the margin-left attribute in the style section.   Once my margin is set, I simply delete the <img> tag that I just added.

Again, this option only works on file names that are a single word. Also, this method of finding out where to place the margin will only be useful if the image you use does not cover the entire width of the screen, which is the most common way of creating an image for stationery.  Many people create background images that have a left margin image as well as a plain background that covers the rest of the screen.  As you might well imagine using this trick to determine the margin with such a graphic is not a good idea.