I put the code I gave you...with minor modification (below)... into sigil and came up with the following for Sigil and ADE...seemed to work fine. I've also attached the actual test epub if that helps.
CSS:
Code:
div.FrameSmall {clear:left; width:100%}
div.Float-L {float:left; margin:0 15px}
p {text-align:justify; margin:0 0 .5em; text-indent:0}
HTML:
Code:
<body>
<p>...and is represented chemically as CH4. It is often shown pictorially as:</p>
<div class="FrameSmall">
<div class="Float-L"><img alt="" src="../Images/Methane15-1.png" /></div>
<p class="First6">Methane where the single lines drawn between the hydrogen (H) and the carbon (C) atoms denote the sharing of a single electron pair. If two electron pairs are shared as often happens when atoms bond with oxygen for example, a double line is drawn, and so on.</p>
</div>
<div class="FrameSmall">
<div class="Float-L"><img alt="" src="../Images/Ethane15-2.png" /></div>
<p class="First6">Carbon can also share a single electron pair with another carbon atom, with hydrogen providing the remaining three available bonds. In such a case, the resulting compound is ethane (C2H6), which is depicted on the left.</p>
</div>
<p>Methane and ethane are but simple compounds ...</p>
</body>
edit: oops...I added an extra "Methane" to the paragraph next to the methane pic.