Converting Image Files to PDF without third party

And? You are waiting for permission, perhaps? If so, then consider it given.

If not, then you will have to read up on the file spec for PDF files[^], and implement the relevant sections.

The you have to study the PDF specifications and implement yourself the algorithm.
The latest specifications available for free are the 1.7[^].

Well, good luck then. First of all, you are going to need a copy of the PDF specification[^]. Then you are going to need an awful lot of time, patience and caffeine.

Why do you need to write all this for yourself? There are several free libraries that you could use to perform this for you - without the headache of having to understand, write, test and bug-fix.

Raster images can't reasonably be 'converted' to PDF (without lots of clever code to recognise paths, text etc within an image). However, you can include an image in a PDF, and if that's all you want in the PDF, it is a fairly trivial one. JPEGs can be included verbatim, other formats you will have to construct the bitmap in simple 32 bit (might be 24, it's a while since I did this) format and then optionally compress it (gzip, use SharpZipLib for that in my opinion, I know that's third party but it's LGPL and good) before including it.

Vector images can be converted to PDF, and since vector image formats are generally based on PostScript, it is usually relatively easy to do so.

To create a PDF you need to understand the file format. Griff posted a link, but I recommend looking at one of the earlier versions of the spec located here[^], as the fewer features you have to learn the easier it is to create a working document. PDF 1.3 contains everything you are likely to want.

This may help you.
Add an Image to pdf document using ItextSharp[^]