Add
.categoryListBoxContents {text-align: center;}
to your stylesheet.
This is standard in a stock Zen Cart stylesheet, but your template maker (Template Monster) deleted it, making this and several dozen other kinds of elements align left instead of center.
The row arrangement problem is caused most immediately by the Porsche image trying to float left and getting hung up on the corner of the Mercedes image which sticks down a bit lower than its neighbors. The Custom image in turn is getting hung up on the corner of the Porsche image. It's just the luck of the draw that this happened at the bottom of the page - it could have happened anywhere in this grid.
It is caused by a missing
<br class="clearBoth" />
that should appear after each row of images. TM has for some unknown reason removed this from its version of the file.
You really should ask them to fix this error, because it is an error on their part, not a change you want to make to their template.
If they won't fix it, it shouldn't be too hard to find the correct location in the customized file to edit.