As you probably know, some themes can seems ...ugly... with default XOOPS image-set (all dark themes as example). Single solution for now is deliver images with theme and overwrite default images by own... but it'll give some headache on next theme-change, or, if you allow users change theme and store&use selected theme you'll get problems of "poor design" immediatelly.
Well, these rakes beat my forehead too - and idea was born
"We have $xoops_theme variable, why not use it all blocks in for all possible theme-dependent img src?"
Yes, it can require refactoring all templates in all modules (standard and 3-rd party), bur changes are - small, easy...
And they are not mandatory - old img src without $xoops_theme in filename will use default images, as it happens now
But improved templates will give some good (I hope) advantages - you can change only theme and don't worry about something else.

Comments, please?

I see where you are going. Couldn't you do this by releasing a custom template set with your theme as it is now ?

Yes, I can... and this way we all get another type of problem (???), which I tried to describe here