Patrick--To upload the files first I'd recommend making them something like 650 pixels on the long dimension and save as JPEG compression level 4 or so to keep the file size reasonable (since these are graphs, they don't need to be maximum image quality).
Then enter your reply in "advanced" mode and scroll down a bit, and click on "manage attachments." You'll get a pop-up with a number of boxes for uploading attachments. You can click on "browse" for each box to find the JPGs on your hard drive. When you've located all of them, click "upload" and wait until it says it's finished, then you can close the box, and when you "submit reply" in the reply screen, your images will be attached.