The default swift theme image can be changed from the gray ‘swoop’ to an image of your choice.
This means that when you do not manually add an image to a post, your own default image will show instead of the Swift Theme’s image.
The image you want to substitute is located here:
Open the theme’s files. I do this through cPanel, other use something like filezilla.
So for me, in cPanel, I would go to 'public_html', then 'my site.com', then 'wp-content', then ‘Themes’, then your version of Swift.
After opening the Swift Theme files you will see a folder named ‘Images’ and the one you want to edit/replace is the ‘default.jpg’
The ‘default.jpg’,in my version, says, “PNG image data, 580 x 290, 8-bit colormap, non-interlaced”.
Make sure you stay within the dimensions of the original when replacing the image and make sure you have it named the same as the original ‘default.jpg’. You can play around with dimensions under 580 x 290, but I would be careful going larger than those parameters.