Lin,
First, there are two issues at play here: "posting" or attaching a photo, (a function of this forum) and second, converting a photo to the proper size (that's your responsibility).
To attach a photo, click attach at the bottom, click choose file, and browse to the location on your computer where the file is located. You'll note that the file limits and restrictions are quite clear here: 4 attachments per post, total size 8MB (8192KB), each file no larger than 4MB. That's pretty good and has been raised from years past.
Now if you take a high resolution photo these days, it is pretty typical that you'll get one larger than 4MB. Even my phone takes photos larger than that. There are many ways to reduce the size of JPGs, using a number of different software applications (photoshop, photoshop elements, iPhoto, etc.) Personally I use photoshop elements, but there are ways to do it directly on a phone with an app, and there are some free applications for your computer too.