You can use any image hosting facility, e.g. Tinypic, Photobucket or similar. The following applies to Tinypic, but should be similar with others.
1. Go to tinypic.com. You will see a screen like this:

2. Click on the browse button on the tinypic screen and navigate to where you have stored your image. your screen will now look something like this.

3. Click "open" when you have selected image, then "Upload Now" on the tinypic screen.
4. Once you have done that, this screen will apear

Select the text in the IMG Code box, press Ctrl C or right click-copy on it to copy the text then go to the post you are making at Ifboat.com and press Ctrl V or right click-paste to paste it into your post.
Hope this helps!