There is no definitive answer to this question, as it largely depends on the blog platform and specific rules of that platform. However, most blog platforms allow for covers to be posted, with some exceptions.
For example, WordPress does not allow for book covers to be posted, while Google+ does. It is important to check the blog platform’s guidelines before posting a cover to ensure that it is allowed.
If a cover is allowed on a blog platform, then it is generally recommended that covers be cropped to fit within the layout of the blog. For example, if a cover is posted on a blog that has a fixed width layout, then the cover should be cropped so that it fits within that width.
Additionally, certain elements of a book cover may not be allowed on a blog platform—for example, excessive watermarks or graphic designs may not be allowed on many blogs. In these cases, it may be necessary to remove these elements before posting the cover.
Overall, it is generally safe to post book covers on a blog platform provided that the guidelines of that platform are followed.