The standard FBReader software (e.g. on a desktop PC) does almost nothing with images. It always centers them and displays them full size (ignoring all styling directives). If they are too big for the screen, the bits outside the screen view get clipped. I have seen it reported that PocketBook dithers images to 16 greyscales, which is more than a generic port of FBReader would do, but I have not seen anything about added image scaling capability.
Adobe Digital Editions by default does little more than FBReader, and it does not even center images by default. However, ePub documents can include explicit scaling information for images. Images can be scaled to X% of the full width or X% of the full height (but not both at once). Also, SVG (Scalable Vector Graphics) is supported and it allows explicit scaling to both the width and the height at once. You can also embed an image in a SVG to get optimal scaling. The problem is that all this scaling "magic" only happens if the current ePub document includes it. However, this does mean that all the machinery required is available if an implementation of mobile ADE chose do more with default images.
Note that FBReader's automatic centering of images isn't always a good thing. A drop caps letter at the start of a chapter is often an image, and this does not work well with FBReader. Also FBReader does not support SVG graphics, but any DRM-free ePub could be run through Calibre to convert SVG to standard images.
I don't think Calibre can do this at present, but it may be your best bet (in the long term, with new features) for producing optimal sized images. It already "knows" the size of the screen for your target device and can create images sized for that screen. The difficultly is in knowing when to change the size of an image and when not to do so.