Since you have only a free membership, the photos you put in your portfolio and then post here will always be the small size that we are seeing. This size is too small and usually too compressed to do justice to the photos.
You need to post the photos to a web server somewhere, and then link the image from there. Image size of 550 pixels high and 50-80k seems to work best. Here is an example:
If you right click on the image and then click on properties, you'll be albe to see the link and the sizes.
By the way, with a paid membership, you can have larger photos that are just right for viewing in you portfolio and can have 55 of them.