Simple floating images can be like
<img src="yourimage" align="left" />
BUT that way you won't get solid results with padding between text and image, outlook removes margin and padding and your text will stick right next to the image. So try this:
<div style="text-align:justify;">
...a lot of text here untill you want to insert an image that floats left...
<table cellpadding="0" cellspacing="0" align="left" style="float: left;">
<tr>
<td>
<img src="yourimage" align="left" vspace="4" />
</td>
<td width="15"> </td>
</tr>
</table>
...a lot more text here until you need an image that floats right...
<table cellpadding="0" cellspacing="0" align="right" style="float: right;">
<tr>
<td width="15"> </td>
<td>
<img src="yourimage" align="left" vspace="4" />
</td>
</tr>
</table>
... a lot more text here...
</div>
You need to wrap a 'table' element around it to get the padding-margin effect to work in Gmail, Outlook (online), Microsoft Outlook (desktop client),...
Give the table an align=left or right attribute. (Edit answer here: in addition and fallback for other email clients also give the table a float value so do both. They are back-ups to each other. Some clients understand "float", others understand "align", some understand both,...) Your table will float in the text almost like an image does. The only difference is that in outlook a table generates an automatic line break in the text where an image with align left or right does not generate breaks.
For setting the margin, since we are now working with a table, add an extra "td" with a width="15" to the left or right of your image cell and a non-breaking-space in it. (or a transparant gif -> spacer.gif)
You better not leave cells empty because otherwise the width of your cell will not be respected in certain email clients
For top and bottom margin we can use the 'vspace' attribute, don't forget to give the image an align = left or right attribute. Otherwise the 'vspace' will not work.