Aunque la página esté en horizontal, -con las imágenes metidas en una tabla, por ejemplo- las irás llenando en orden (de arriba a abajo y de izquierda a derecha), y ese debería ser el orden en que se carguen.
sobre el código... puedes hacerlo con tablas, con div, ...
Aqui un ejemplo con tabla:
Código:
<body>
<table width="500%">
<tr>
<td width="3%" align="center">1</td>
<td width="3%" align="center">2</td>
<td width="3%" align="center">3</td>
<td width="3%" align="center">4</td>
<td width="3%" align="center">5</td>
<td width="3%" align="center">6</td>
<td width="3%" align="center">7</td>
<td width="3%" align="center">8</td>
<td width="3%" align="center">9</td>
<td width="3%" align="center">10</td>
<td width="3%" align="center">11</td>
<td width="3%" align="center">12</td>
<td width="3%" align="center">13</td>
<td width="3%" align="center">14</td>
<td width="3%" align="center">15</td>
<td width="3%" align="center">16</td>
<td width="3%" align="center">17</td>
<td width="3%" align="center">18</td>
<td width="3%" align="center">19</td>
<td width="3%" align="center">20</td>
<td width="3%" align="center">21</td>
<td width="3%" align="center">22</td>
<td width="3%" align="center">23</td>
<td width="3%" align="center">24</td>
<td width="3%" align="center">25</td>
<td width="3%" align="center">26</td>
<td width="3%" align="center">27</td>
<td width="3%" align="center">28</td>
<td width="3%" align="center">29</td>
<td width="3%" align="center">30</td>
<td width="3%" align="center">31</td>
<td width="3%" align="center">32</td>
<td width="3%" align="center">33</td>
</tr>
<tr>
<td width="3%" align="center">34</td>
<td width="3%" align="center">35</td>
<td width="3%" align="center">36</td>
<td width="3%" align="center">37</td>
<td width="3%" align="center">38</td>
<td width="3%" align="center">39</td>
<td width="3%" align="center">40</td>
<td width="3%" align="center">41</td>
<td width="3%" align="center">42</td>
<td width="3%" align="center">43</td>
<td width="3%" align="center">44</td>
<td width="3%" align="center">45</td>
<td width="3%" align="center">46</td>
<td width="3%" align="center">47</td>
<td width="3%" align="center">48</td>
<td width="3%" align="center">49</td>
<td width="3%" align="center">50</td>
<td width="3%" align="center">51</td>
<td width="3%" align="center">52</td>
<td width="3%" align="center">53</td>
<td width="3%" align="center">54</td>
<td width="3%" align="center">55</td>
<td width="3%" align="center">56</td>
<td width="3%" align="center">57</td>
<td width="3%" align="center">58</td>
<td width="3%" align="center">59</td>
<td width="3%" align="center">60</td>
<td width="3%" align="center">61</td>
<td width="3%" align="center">62</td>
<td width="3%" align="center">63</td>
<td width="3%" align="center">64</td>
<td width="3%" align="center">65</td>
<td width="3%" align="center">66</td>
</tr>
</table>
</body>
