Hi Nivea,
U've written a quite good letter except for a few grammatical errors. I reorganized some of the setences. The meaning has been changed a bit. If Dr xxx belong to the organisation, if so, the Dr will represent the organisation. you can use 'your organisation' to address the organisation. Second, are the photos included in those letters(emails)? I'm not sure whether my interpretation accords to your original intention. If not, you just change back to your original organisation.

As the personal assistant of Chairman xxx, I've already sent email to xxx website several times, however, we haven't gotten any reply from xxx official organisation yet.

Since there's little time to the due date, I'm a bit worried and very anxious to know what has happened to those enquiring letters. We have sent four photos through these emails' attachments for making security cards. Meanwhile, we hope to receive the invitation letter from you for this meeting through email or fax as soon as possible. We need it as an important document for visa application.

I look forward to hear from you soon.