It was kind of a fluke. The Texas Environmental Enforcement Dept. puts up cameras at spots that are popular for illegal dumping. The officer was reviewing the pictures, and of course that one caught his eye as she was dumping something with a chemical warning stamp on it. He had caught the news and put 2 and 2 together when he saw a picture of Amanda and Grant. He then subsequently turned the photos over to LE.
Right now, the illegal dumping charges are on hold until after a verdict in her murder trial, so she can still be convicted of that too.