To insert pictures, you can click the handy "Image" button above the question/answer field:
This will prompt you to locate the picture either on your computer or on the internet. It will upload it to StackExchange's special imgur account1 and insert the appropriate formatted link in the text field. Note that you are encouraged to replace the highlighted text with some image description.
As for making diagrams, there is no particular toolset on the site.2 We do have many suggestions, though, for standalone tools. Use whichever you want, and upload the result.
Why nothing integrated into the site? You can make a request, but as you'll see in that last link, there are many different opinions about what works best. Besides, whatever option there is has to be amenable to integrating into websites.
1 You should do this rather than just format an appropriate link to the image. This way the picture is permanently stored even if there is link rot.
2 Unlike, for instance Electrical Engineering, which has a tool for making circuit diagrams.