You can and are encouraged to use your own pictures.
Check out these three tutorials:

1. A few ways to post pictures on Steemit, https://imgsafe.org/ being the easiest and best way i've found so far. https://steemit.com/steemit/@kakradetome/a-few-ways-to-post-pictures-on-steemit-https-imgsafe-org-being-the-easiest-and-best-way-i-ve-found-so-far
2. Steemit - Tutorial - This is how to upload pictures/videos https://steemit.com/new/@xavier1/steemit-tutorial-this-is-how-to-upload-pictures-videos
3. How to place images, links and videos in STEEMit? https://steemit.com/steem/@coar/how-to-place-images-links-and-videos-in-steemit