Your image attached shows its size in kilobytes & pixtels :54.61 kB, 248x187
Your computer is displaying at the appropriate size to give an acceptable image quality. If your images are of this quality , you will not be happy to 'blow them up' bigger.
If you can take photos at a higher resolution, then larger images will be acceptable. Hope this helps.
This site does not accept high resolution images due to data restriction
As advised:
"Allowed file types: txt, jpg, gif, pdf, mpg, png
Restrictions: 30 per post, maximum total size 5000KB, maximum individual size 3000KB"