Maybe the problem is with the image, are you sure it has a minimum size to fit where you want? And if it has the quality you need? Steemit probably have a max image size, so it may be cutting your file, if the image you want to use is too big maybe you should host it somewhere else and link the file to your post.
FROM STEEMIT FAQ:
"How do I add images and photos to my posts?
You can browse your hard drive to add an image by clicking on the "selecting them" link from within the editor.
If you have an image copied to your clipboard, you can simply paste (ctrl + v) while in the post/comment editor, and your image will be uploaded into your post or comment. Due to the file size of these pasted images, this method is only recommended for simple graphics. Photos (.JPG) should be uploaded from your disk.
Pictures can also be hosted on an external site. Paste the image's web address (URL) into the editor and it will automatically be added.
How do I set the thumbnail image for my post?
The first image in the post will automatically be set as the thumbnail image.
What is the recommend aspect ratio for thumbnail images?
The recommend aspect ratio for thumbnail images is 16x9."