The Kindle Publishing Guidelines indicate the the following html attributes of the img tag are supported by the Kindle: align, border, height, id, src, width. The following attributes are not: alt, hspace, longdesc, lowsrc, title, usemap, vspace. The vertical-align property in your CSS may not be able to be converted by Kindlegen (Kindle Previewer).
You can try using the html attributes (basically any HTML 3.2 attributes) directly in the source and see if it makes a difference. I know you can't "wrap" text around images in a mobi with floated divs. You may have to experiment with creating a simple table; one cell for the image and one for the text, but beware... anything other than the most simplistic of table layouts/formatting will probably not work very well. especially on smaller devices.