Is it worth hiding an AES encrypted text string in a PNG image for better security?
"Hiding" content in an image (or other medium, it's a general technique called steganography) can be done either in a very obvious way (a dedicated data block, called ancillary chunk, which does not form part of the image), or it can be more effectively hidden in the low order bits of the image, thus very subtly degrading the picture. 

Both methods can be discovered by a dedicated analyst, there are even automated tools which can flag images if some telltale statistical anomalies are detected.

This method doesn't add any extra security for the secret message - which is already encrypted anyway. The only thing it provides is a very weak protection of the fact that a secret message existed at all.