Hi Cristina,
You can usually post JPGs images to the forum (and pretty much any forum) because the size of that image format is usually small enough for forum use.
When creating a new post or replying to one, you basically go to the "Add an Attachment" and in the
Filename row, use the "Browse" button to access files from your hard drive (or other file source on your end). Go to the subfolder with the JPG image you would like to upload and then hit "Open". Then the image location path is put in the "buffer", ready for uploading. You can then add a file comment for the image in the
File Comment text box below it, or simply finish composing your main text message. After that, hit "Submit", and your message, along with the image, will be uploaded to the forum either in the existing thread you responded to or in a new thread (that is, if it is a new message).
Well, this response is somewhat late, but hope it helps some.
Cya.