There are two ways that I have found. One is to use the attachment feature, which has never given me the results I want. The other is to put the photo on some photo server and link to it.
There is a blue area at the bottom of the dialog this brings up. It has two tabs, one is labeled "Attachments". You click this and a button appears that says "Add file". You click this and your browser should open a gizmo that lets you rummage around on your local machine, find the file you want to attach and then "Open" it to attach the file. You will get a complaint if the file is too large. I could experiment by trial and error to find out, but I don't have the energy. A 640 pixel wide file (256K) works. A 1024 pixel wide file (563K) works. No telling if the size limit is based on pixels or size in bytes.
I just experimented with a 1024 pixel wide file. The forum inserts a tiny thumbnail (400 pixels wide) no matter what the size of your image. If you click on the thumbnail, you get a page that shows the full size image. It seems that nobody on the forum uses this feature.
I resize my photos (in lightroom) to 640 pixels wide before downloading them to postimage, but I see people saying that you don't need to resize your photos first. I wonder what happens if you resize to 1024 pixels and download to postimage? Well, in short, it works even better!
I tried a 1024 pixel wide image and it made it to the forum 1024 pixels wide by way of "postimages". This actually seems the perfect width, it neatly fills the width the forum allocates for text.
Tom's Knife Info / tom@mmto.org