Your validation is referring to the upload file object before it is created.
Instead of:
$WAFV_Errors .= WAValidateNM($WA_DFP_UploadStatus["WA_UploadResult1"]["WA_UploadResult1_1"]["fileSize"] . "",0,2097152,"",",.",true,1);
you can use:
$WAFV_Errors .= WAValidateNM($_FILES["PgMainPic"]["size"] . "",0,2097152,"",",.",true,1);
The show if text will show until a valid form is submitted once it is shown. You could add a url parameter to clear it, or even clear it manually at the bottom of the page like:
<?php
@session_start();
$_SESSION['WAVT_apagesmainpic2_085_Errors'] = "";
?>