I've just changed the upload limit to make it possible. Around the insert image button, you'll see a little paperclip - same as the one in Outlook. Click that, and click "add files" right up the top of the pop up box. Click select files, browse to the PDF of your computer and then click "upload files". Then click insert attachment, and people will be able to download the PDF from a link in your post.
Max file size is 5mb, I don't really want to increase this but if you have something that is 6mb or something I could be persuaded. If it's way larger you can just push print screen on your keyboard, which will copy your screen to your clipboard. Then go to an image editor like paint (or photoshop etc), paste it in there and save it as a jpeg.
Edit: I was responding to the PDF question above but the process for jpegs is the same. I've just changed the settings so that when you upload them via the attachment option, they will be displayed inline as below.