You cant just save the image as a bmp if its a JPG. Today the web format for images is jpg, gif and png, so i suppose the forum will not accept BMP.
What you need to do before uploading is to convert the images to jpg before uploading with an image editor software. You dont have a image editor? No problem. Just open Paint that comes with yout Windows OS. On paint open the image ( you should first save it somewhere on your pc, you cannot open directly from the mail), paint will give a message that want to resize the field, just say yes. After that just go to save as on the paint menu, and on the list choose to save as JPG, or JPEG (its the same)
It will give you again a warning about losing quality just give it yes again. Now you have the image on a jpg format. You can upload the image now. I hope this helps.