Ummm ... because I don't know how to do that?
When you're done typing your post, click on "Preview." Scrolling down past the preview/edit window, you'll find "Additional Options." Clicking on that gives you the opportunity to add attachments, within the size and number limitations. See attached screen shot of that below.
The trick is getting your image down to the 128KB limit, and that involves using MS Paint or another image editor to reduce the size, maybe crop out extraneous stuff, and save as JPG or JPEG, which saves space. Then store the reduced image someplace you can find easily, and attach it per the attached image below by hitting browse and going to that known storage spot and selecting that image.
Works pretty good for me, and I don't use any hosting site, but I'm pretty adept at fitting a pic into the attachment limitations on APS. I reduced this one all the way down to 38 KB, and it's still look-at-able, even at that reduced size from a full screen shot.
Oh, one hint. Be patient in letting the various softwares do their things before clicking "Save" or going to the next step.
Terry, 230RN