There is nothing wrong in showing a picture that you liked.
However, credits are necessary on an image if it features material that you have not made.
You can easily edit the image in Photoshop and say:
Insignia class ship by Mark Kingsworth,
Image by 'Gorgon'
Fairly simple to do and takes only a few mins.
Generally speaking, if you post images that contain material only you made, then credits are not needed, however, it IS encouraged in order to avoid others stealing your work.
I had several people over the years ask me if they can use my images in various projects.
I told them: 'Sure, so long as the said images still contain the credits I originally put on them'.