Hello;
replace the previous code by this one, if you want to have 30 pixels width and height por your previous and next thumbnails:
<td class="row2"> {if prev_image_name}{lang_prev_image}
<b><a href="{prev_image_url}">{prev_image_name}</a></b>
<br /><br /><a href="{prev_image_url}"><img src="{prev_thumb_file}" width="30" height="30" border="1"></a>
{endif prev_image_name} </td>
<td align="right" class="row2"> {if next_image_name}{lang_next_image}<br />
<b><a href="{next_image_url}">{next_image_name}</a></b>
<br /><br /><a href="{next_image_url}"><img src="{next_thumb_file}" width="30" height="30" border="1"></a>
{endif next_image_name}</td>
Hint: if your pictures are not squared, try to use only the height in pixels, to keep natural looking thumbnails. The width of the thumbnail will be resized proportionnally to the height.