The Sun quotes a source saying the star fired her PR adviser because she was 'furious' that the fallout would affect Amanda's career.
But a spokeswoman for the 44-year-old Britain's Got Talent judge told the Mail that the aide was fired 'solely due to her being so devastated that a recent story involving her daughter went out of control and sick comments were being made towards her nine-year-old'.
The picture of Lexi in a strapless red dress was posted on Amanda's official Twitter account on February 13 with the caption: "1990's day at Lexi's school today. Julia Roberts 'Pretty Woman'."
Amanda then blamed her husband Chris Hughes for publishing the picture and denied the schoolgirl was dressing as fictional prostitute Vivian.
The Sun's source said: "As far as Amanda is concerned, this slip-up came at exactly the wrong time and she is really worried it will affect her career.
"She is in talks with ITV about future TV projects at the moment, so is keen to appear whiter than white rather than tarnished by this photo.
"When Amanda realised how bad the fall-out was, she was absolutely furious and since then, she and her aide - who she has been with for years - have not been working together.
"Apparently she blamed her for not handling the situation properly.
"But their parting of ways - especially after such a long time - has stunned a lot of people.
"It has been pointed out that there was very little the aide could have done to halt the backlash.
"Once the photo was out there, it was out there."
However, Amanda 's spokesman told the Mail: "This story is untrue. Amanda's change in PR agencies has nothing to do with any fears over her career, but solely due to her being so devastated that a recent story involving her daughter went out of control and sick comments were being made towards her nine-year-old.
"She felt that the situation was underestimated and not properly handled resulting in Amanda having to take legal action to ensure her child's safety."
After the image was republished on numerous websites and in national newspapers, Amanda threatened to sue for breach of copyright if they continued to use the picture.
She then released a statement insisting Lexi was dressed as Cindy Crawford.
In a statement, Amanda said: "The truth is that Lexi went to school as Cindy Crawford for her school '90s Day - not Julia Roberts - as anyone who heard me yesterday at BGT - and indeed the teachers at Lexi's school - will confirm.
"However my husband thought she looked more like Julia Roberts, without any thought to any interpretation. Although given that she is a nine year old girl it seems completely unbelievable and disturbing that anyone could or would put such an interpretation on it.
"Julia Roberts, is one of the most beautiful and iconic women in the world and so, as a proud father, he wanted to post the similarity on his own private Facebook account. He hit the wrong button and posted it on my twitter account- but removed it as soon as he realised.
"The first I knew about it as I was contacted by the media whilst filming today. Needless to say my husband is mortified."