How has the publisher specified the image size in the stylesheet?
If they have used an img style with a height and width of auto or height and width not specified at all then the image will display at its natural pixel size with no scaing. In that case you could easily scale up all the images by exploding the book in Calibre and running an image manipulation tool in the book's images directory. For example in Linux with the image magic tools you could make all JPG images 50% bigger by running:
Code:
for F in *.jpg; do convert -resize 150% $F $F; done
But really, all images in an ePub should be displayed at a size proportional to the font size (em), or screen size (%), or in some rare cases absolute size (mm, pt, etc.) Any publisher that displays the images in pixel (px) sizes is just ensuring that the images will be the wrong size on every device with a pixel density different to the one they tested on. The only way to fix that sort of mistake properly is to go through the book and correct each individual mistake. by hand.