Since you are not a paid member, the images in your portfolio here on GG will be limited in size and, thus, viewability.
For you, the best way to get a good viewable image would be to have them hosted on a site outside of GG (for example, if you have web space from your ISP that you're not using, you can still put pictures there and link them to your messages, here). For example, Charter Cable is my ISP, and I get a free 20 megabyte web space that I use only for hosting images... you might have a similar deal, or you may choose one of the "free" or low-cost image hosting options out there.
To post the image, look for the "image" UBB code in the little box right below where you compose your message. Click on image, and a window pops up asking you for the link to your image. Put in the web address like this:
That make sense?