Might be related with
http://bugs.php.net/bug.php?id=42266

this just adds a record to the long list of firebird blob related bugs introduced after 5.2.1

Please try using this CVS snapshot:

  http://snaps.php.net/php5.2-latest.tar.gz
 
For Windows:

  http://windows.php.net/snapshots/

No feedback was provided for this bug for over a week, so it is
being suspended automatically. If you are able to provide the
information that was originally requested, please do so and change
the status of the bug back to "Open".

Here is the code:

function glGetPic2()
{
  $sql = "select
 a.id, a.descri, a.url, b.purpose, a.pic, a.mime
from glpics a
left outer join picpurps b on b.idpurp=a.idpurp
where a.iden = ".$_REQUEST["iden"]."
and a.idtabla = ".$_REQUEST["idtabla"];
 $con = ibase_connect("glpics","sysdba","kliam56") or die(ibase_errmsg()." No se pudo conectar a la base glpics");
 $res = ibase_query($con,$sql);
 while($dat = ibase_fetch_row($res))
 {
   echo "<div style='width:65%;margin:0 auto 0 auto;' >";
   if(strlen($dat[2]) > 0)
   {
     // se trata de un URL
     echo "<img style='width:90%;display:block;margin:0 auto 0 auto' src=\"".$dat[2]."\" />";
   }
   else {
     // se trata de un blob
    /* $bi = ibase_blob_info($dat[4]);
     $ln = $bi["length"];
     $bo = ibase_blob_open($dat[4]);
     $pblo = ibase_blob_get($bo,$ln);
     ibase_blob_close($bo);
     $fp = fopen("/home/lazaro/public_html/img/glpic".$dat[0].".jpg","w");
     fputs($fp,$pblo,$ln);
     fclose($fp);
     echo "<img style='width:90%;display:block;margin:0 auto 0 auto' src='img/glpic".$dat[0].".jpg' >";*/
    header("Content-Type: ".$dat[5]);
    ibase_blob_echo($dat[4]);
    
   }
   echo "<p>".$dat[1]."&nbsp;(".$dat[3].")</p></div>";
 }
}

As I receive the data, it comes in the form of characters from the content of the blob. My text is properly inserted. It is received through ajax, but the result in a direct URL is just as wrong, except that it wants to save the page.

It definitely doesn't show the images. They're all jpg now.

@ffortuny:
This is not the place to get help with your code. Please take your questions to a mailing list or online forum, where they will tell you about how you can't output HTML and image data at the same time.
http://www.php.net/support.php