これを試してみてください
// if your src has widthxheight are specified literally like that you may try
echo preg_replace("/\W{0,1}(width).*(height)/i","-300x300","http://www.mysite.com/myfolder/files/year/month/imagename-widthxheight.imageextension");
// if your src has widthxheight are specified in int val you may try
echo preg_replace("/\W{0,1}(\d{1,7}).*(\d{1,7})/i","-300x300","http://www.mysite.com/myfolder/files/year/month/imagename-123x456.imageextension");
-300x300の実際の値は実際の要件によって異なります。だから私は変数を介してこれらの値を渡すのが最善だと思います。