At the bottom of the post there is an option for attachments. However, you many need to fix some possible issues:
1. File name may be incapable with this site. Right click on the file name and select "Rename". Change to something like Car1. Careful in that sometimes you delete the extension ".jpg" and may need to add it again.
2. The file can be too big. Right click on the file, select the option "Open with" Choose: Microsoft Office. At the top middle select edit, and menu will open on the side, choose "Resize" and they put in some percent, 20-30% is normally OK to get files around 100KB which is a good size for posting.
3. If your file extension is ".bmp" it will probably not be accepted. Again choose "Open with" and then Paint. Go to "save as" and choose the jpg extension, then hit save.