The image you posted is the primary source of value for your post or you have posted an image in a curated and/or artistic tag (photography, art, etc.) without attribution or source. By doing so you are claiming to be the sole content creator and that the image is original to yourself.
Plagiarism is the copying & pasting of others' work without giving credit to the original author or artist. This includes manually reproducing the work of others, which violates their intellectual property rights.
Repeated plagiarism is considered fraud. Fraud is discouraged by the community and may result in the account being Blacklisted.