Lazy Gallery doesn’t have valid XHTML

I’ve tried searching for a similar topic in these forums, but I can’t seem to find anything, so here goes:

I’m using the Lazy Gallery plugin by lazyboy.dk, and I noticed that the pages generated by this plugin generate a lot of errors in the XHTML checker.

I was able to fix some of these errors myself, such as the missing closing tags for br and img properties. However, I also get the folllowing PHP-related errors:

Line 91 column 479: cannot generate system identifier for general entity “thumb”.
Line 91 column 479: general entity “thumb” not defined and no default entity.
Line 91 column 484: reference not terminated by REFC delimiter.
Line 91 column 484: reference to external entity in attribute value.

Unfortunately, my PHP-skills are virtually non-existant and I have no idea what these errors mean or what I’m supposed to do about them.

Does anyone else use this plugin and know how I can fix these errors?