Did not notice this in Internet Explorer, but in two other browsers (Google Chrome and Apple Safari) I get a weird effect, such that the (tabled) content of my posts is coming through the sidebar.

See for yourself at:
http://www.elefantin.nl/

Any idea why that is happening?

try adding 'size-full' to the 'class=' in the <img.....>

Please can you be a little more precise?

Did you mean adding an attribute to the CSS class, or adding something to the <image ...> tag itself?

Also I do not know about a 'size-full' attribute.

If you meant size="full" then my CSS editor gives an error saying that size is not an allowed attribute in XHTML transitional 1.0

Are you using a plugin to add the images, or did you build teh table yourself?

If you built it yourself, you have this code

<table class="photoblog" border="0">
<tbody>
<tr>
<td><a href="http://www.elefantin.nl/gallery/sprookjesbeelden/index.html"><br />
<img class="style_no_border" src="/blogged/blog_sprookjesbeelden/p1.jpg" alt="sprookesbeelden1" width="180" height="135" /></a></td>
<td><a href="http://www.elefantin.nl/gallery/sprookjesbeelden/index.html"><br />
<img class="style_no_border" src="/blogged/blog_sprookjesbeelden/p2.jpg" alt="sprookesbeelden2" width="100" height="135" /></a></td>
<td><a href="http://www.elefantin.nl/gallery/sprookjesbeelden/index.html"><br />
<img class="style_no_border" src="/blogged/blog_sprookjesbeelden/p3.jpg" alt="sprookesbeelden3" width="180" height="135" /></a></td>
<td><a href="http://www.elefantin.nl/gallery/sprookjesbeelden/index.html"><br />
<img class="style_no_border" src="/blogged/blog_sprookjesbeelden/p4.jpg" alt="sprookesbeelden4" width="100" height="135" /></a></td>
<td><a href="http://www.elefantin.nl/gallery/sprookjesbeelden/index.html"><br />
<img class="style_no_border" src="/blogged/blog_sprookjesbeelden/p5.jpg" alt="sprookesbeelden5" width="180" height="135" /></a></td>
</tr>
</tbody>
</table>

just change each of the class="style_no_border" to class="style_no_border size-full". The 'size-full' class is built into the Atahualpa CSS

If a plugin created the table, you could try a CSS Insert

table.photoblog tr td a img.style_no_border{
max-width:96%;
width:auto 100%;
margin:5px 0 5px 0
}

I created the HTML myself, and adding the extra class info makes the images to disappear completely.

did you make it look like this

<table class="photoblog" border="0">
<tbody>
<tr>
<td><a href="http://www.elefantin.nl/gallery/sprookjesbeelden/index.html"><br />
<img class="style_no_border size-full" src="http://www.elefantin.nl/blogged/blog_sprookjesbeelden/p1.jpg" alt="sprookesbeelden1" width="180" height="135" /></a></td>
<td><a href="http://www.elefantin.nl/gallery/sprookjesbeelden/index.html"><br />
<img class="style_no_border size-full" src="http://www.elefantin.nl/blogged/blog_sprookjesbeelden/p2.jpg" alt="sprookesbeelden2" width="100" height="135" /></a></td>
<td><a href="http://www.elefantin.nl/gallery/sprookjesbeelden/index.html"><br />
<img class="style_no_border size-full" src="http://www.elefantin.nl/blogged/blog_sprookjesbeelden/p3.jpg" alt="sprookesbeelden3" width="180" height="135" /></a></td>
<td><a href="http://www.elefantin.nl/gallery/sprookjesbeelden/index.html"><br />
<img class="style_no_border size-full" src="http://www.elefantin.nl/blogged/blog_sprookjesbeelden/p4.jpg" alt="sprookesbeelden4" width="100" height="135" /></a></td>
<td><a href="http://www.elefantin.nl/gallery/sprookjesbeelden/index.html"><br />
<img class="style_no_border size-full" src="http://www.elefantin.nl/blogged/blog_sprookjesbeelden/p5.jpg" alt="sprookesbeelden5" width="180" height="135" /></a></td>
</tr>
</tbody>
</table>