Kobo ignores embedded fonts in kepubs but not epubs. If the font the Kobo software decides to use in place of the embeded font is significantly different in proportions it will result in text strings overlapping each other and text being truncated at the right side of the image if the default font is wider than that used in the image. This applies to Kobo PC, Kobo Android and Kobo readers.
I'm currently struggling with SVG images in my books being rendered as pure black rectangles on my Kobo Touch reader (firmware 3.3.0) when viewed as kepubs. But not all images, just some, and they all contain a lot of text. So far, I have not found anything that is common only to the images that are rendered as black rectangles. Some of the images that display properly also have a lot of text. I didn't have this problem until I updated my firmware.
You can get your images to scale by setting width or height to a percent less than or equal to 100% and setting the other dimension to auto. Do it inside the <img> tags where you probably now have the height and width set to a fixed number of pixels. For example: width="90%"; height="auto"; You should make the smaller dimension auto in most cases.