why do my pages come up with validation errors whereas other zenphoto sites I've seen do not?

The error appearing are all to do with i.php:

e.g.
<img src="/gallery/zen/i.php?a=cp2&i=DSC02539.jpg&s=thumb"... etc

Warning Line 21 column 119: cannot generate system identifier for general entity "i".
Error Line 21 column 119: general entity "i" not defined and no default entity.
Warning Line 21 column 120: reference not terminated by REFC delimiter.
Warning Line 21 column 120: reference to external entity in attribute value.
Error Line 21 column 120: reference to entity "i" for which no system identifier could be generated.
Info Line 21 column 118: entity was defined here.
Warning Line 21 column 134: cannot generate system identifier for general entity "s".
Error Line 21 column 134: general entity "s" not defined and no default entity.
Warning Line 21 column 135: reference not terminated by REFC delimiter.
Warning Line 21 column 135: reference to external entity in attribute value.
Error Line 21 column 135: reference to entity "s" for which no system identifier could be generated.
# Info Line 21 column 133: entity was defined here.

you'd have to replace the & with & in the url's in the following files classes.php and template-functions.php.

`&` that is ...

Oh right. That. It's a minor deviation with no actual impact... we might change it, but no guarantees.