Are there any guidelines for how to distribute images in a (bachelor) thesis?
The basic guideline for this should be your institute's guideline on how to layout bachelor or master thesis.
From a typographic standpoint you should never mix one column with two column layouts. LaTeX offers packages which you can use to wrap text around figures to reduce white spaces.
Like you said and did, I also would put the figures around the spots where they are mentioned in the text. LaTeX arranges them automatically (you can however specify some positions in your figure environment). But from my experiences, it does a very good job at finding the right positions.