If kindlegen doesn't work,... (I didn't know it could do that, and had already written this post by the time I saw the previous one, so I posted it anyway
) (also, I don't like kindlegen since it makes files triple the size):
In the calibre library you can use the Edit Book feature (the shortcut key is "T"). Select the .svg files in the file browser, Right click and select "Export _______". Open the files in GIMP or Inkscape, or some other image editor (or use something like
http://image.online-convert.com/), and save them as JPEGs. Then go back to calibre Edit Book, right click on the file you converted, and choose "Replace _____ with file..."
repeat this for each file you need to convert.
Lastly, run the Check Book validator, by clicking on the ladybug icon in the toolbar.
You should get 6 errors about CSS Property: Unknown Property name -- you can ignore those.
The first bunch of errors, however, will relate to the changed picture -- select "The file ____ has a mimetyp that does not match its extension" and select "Try to correct all fixable errors automatically". Save the EPUB, then exit. This should fix the problem.