Hey Nikki-
I looked thru the screenshots you attached and they all seem to have the common thread that the bg-image is just a little wider on each side than the content that's "on top" of it in that banner section. This tells me that Outlook is filling the background image based on the width of the content. One thing that jumps out at me is that your inner table is set to a width of 500 and has a margin: 0 auto; -- you might try setting that to 600px and then nesting a table inside with 50px <td> spacers in place of the left/right margin. These leave less room for Outlook to interpret the width of that section, and the less you ask of Outlook, the happier everyone is 
You might try replacing this:
<table class="deviceWidth" style="width: 500px; margin: 0 auto; text-align: center; border-collapse: collapse;" align="center" border="0" cellpadding="0" cellspacing="0" width="500">
<tbody>
<tr>
<td>
<div id="banner1-textbb67b845-ca21-4207-aff2-8adfd0cb0c05" class="mktoText" mktoname="Banner1 Text">
with this:
<table class="deviceWidth" style="width: 600px; margin: 0 auto; text-align: center; border-collapse: collapse;" align="center" border="0" cellpadding="0" cellspacing="0" width="600">
<tbody>
<tr>
<!-- left "margin" -->
<td width="50" style="width:50px;"> </td>
<td width="500" style="width:500px;">
<div id="banner1-textbb67b845-ca21-4207-aff2-8adfd0cb0c05" class="mktoText" mktoname="Banner1 Text">
...
</div>
<!-- right "margin" -->
<td width="50" style="width:50px;"> </td>
I also saw that you mentioned that it might have something to do with image scaling. Here's a little nugget of code that I include immediately after the <head> element on the template that talks to Outlook and asks it not to make images any bigger than 96dpi. I use it more-so for the 120dpi Outlook settings, but it might work here if you're seeing inconsistent displays of similar code.
<!--[if gte mso 9]><xml>
<o:OfficeDocumentSettings>
<o:AllowPNG/>
<o:PixelsPerInch>96</o:PixelsPerInch>
</o:OfficeDocumentSettings>
</xml><![endif]-->
At the end of the day, we normally recommend against using background images in email b/c it's a proven show-stopper and they're difficult to maintain. You might want to check to be sure that your images are sized exactly to spec for desktop - I've seen images that are too large/small cause erratic display issues like this before. Under the hood, Outlook is doing some "magic" when code hits the inbox and there's some 'fuzzy math' that happens with px values are converted into pc and then back into px for display/rendering. I've also seen situations with bg-images in Outlook where adding a height to the container w/ the image and content (being as explicit about sizing as possible, everywhere) has actually helped to increase the width of that image. I mention that b/c it looks like you're ending up with a "squeezed" image in the screenshots.
The vml code can also be a bit buggy, you've got to remember you're asking a non-native Microsoft document to "speak" MSO and sometime things get lost in translation. This is another thing I try to avoid b/c even when I've thought I've had a "bulletproof" solution, it's ended up back in my inbox enough times for me to abandon the practice.
Hope some of this leads to a solution!
-Dave
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