More details/clarification please...

"whenever the cache clears"

Do you know which cache?

"the custom background image I had set for the theme"

Is this an edit of the 'outer_background' theme image, or a new theme image tied in via a custom CSS edit?

"Analyzing the theme's images shows the custom background image is still set as the outer background image for the theme"

So I think you mean it was an edit of 'outer_background' then? Just somehow when the CSS is recompiled it is picking the wrong URL even though the URL of outer_background was edited and still shows as edited looking at the settings of the outer_background theme image?

I'm not sure which cache it is exactly to be honest.

Yes, it is the outer_background image

I think that may be what is going on. I uploaded a custom image for the outer_background image for the theme, but Composr is picking the default generated outer_background one and loading that instead. My custom outer_background image works after I first create the theme, but once the cache (don't know which one) clears, it starts showing the default outer_background image instead of the one I uploaded for outer_background. But the theme image manager still shows my custom image as the set outer_background. So that leads me to believe you may be right about it picking the wrong URL.

Ok looking at this now.

I think partly related to a cache emptying change in the latest RC. We made it so any emptied CSS and JS files get rebuilt right away, to reduce the chance of concurrency issues. However, I think a script that didn't have a proper DB or forum driver connection instantiated was clearing the template cache, that might have been your code - result was falling back to the default file (not theme wizard, default theme). It's the IMG_INLINE symbol, so it is base64 within the CSS so that obscured a bit what was happening.